Fixes errors in the tool when 1) linking qstrs in native ARM-M code; 2)
freezing multiple files some of which use native code and some which don't.

Fixes issue #4829.

The user can now select their own package index by either passing the "-i"
command line option, or setting the upip.index_urls variable (before doing
an install).

The https://micropython.org/pi package index hosts packages from
micropython-lib and will be searched first when installing a package.  If a
package is not found here then it will fallback to PyPI.

Prior to this patch, when a lot of data was output by a running script
pyboard.py would try to capture all of this output into the "data"
variable, which would gradually slow down pyboard.py to the point where it
would have large CPU and memory usage (on the host) and potentially lose
data.

This patch fixes this problem by not accumulating the data in the case that
the data is not needed, which is when "data_consumer" is used.

The qstr window size is not log-2 encoded, it's just the actual number (but
in mpy-tool.py this didn't lead to an error because the size is just used
to truncate the window so it doesn't grow arbitrarily large in memory).

Addresses issue #4635.

Fixes the regression introduced in ea3c80a5

How to use this feature is documented in docs/develop/cmodules.rst.

This system makes it a lot easier to include external libraries as static,
native modules in MicroPython.  Simply pass USER_C_MODULES (like
FROZEN_MPY_DIR) as a make parameter.

This adds support to freeze .mpy files that contain native code blocks.

When encoded in the mpy file, if qstr <= QSTR_LAST_STATIC then store two
bytes: 0, static_qstr_id.  Otherwise encode the qstr as usual (either with
string data or a reference into the qstr window).

Reduces mpy file size by about 5%.

Instead of emitting two bytes in the bytecode for where the linked qstr
should be written to, it is now replaced by the actual qstr data, or a
reference into the qstr window.

Reduces mpy file size by about 10%.

This is an implementation of a sliding qstr window used to reduce the
number of qstrs stored in a .mpy file.  The window size is configured to 32
entries which takes a fixed 64 bytes (16-bits each) on the C stack when
loading/saving a .mpy file.  It allows to remember the most recent 32 qstrs
so they don't need to be stored again in the .mpy file.  The qstr window
uses a simple least-recently-used mechanism to discard the least recently
used qstr when the window overflows (similar to dictionary compression).
This scheme only needs a single pass to save/load the .mpy file.

Reduces mpy file size by about 25% with a window size of 32.

POP_BLOCK and POP_EXCEPT are now the same, and are always followed by a
JUMP.  So this optimisation reduces code size, and RAM usage of bytecode by
two bytes for each try-except handler.

Under python3 (tested with 3.6.7) bytes with a list of integers as an
argument returns a different result than under python 2.7 (tested with
2.7.15rc1) which causes pydfu.py to fail when run under 2.7.  Changing
bytes to bytearray makes pydfu work properly under both Python 2.7 and
Python 3.6.

If you happen to only have a really simple frozen file that doesn't contain
any new qstrs then the generated frozen_mpy.c file contains an empty
enumeration which causes a C compile time error.

Following an equivalent fix to py/bc.c.  The reason the incorrect values
for the opcode constants were not previously causing a bug is because they
were never being used: these opcodes always have qstr arguments so the part
of the code that was comparing them would never be reached.

Thanks to @malinah for finding the problem and providing the initial patch.

A DFU device must be in the idle state before it can be programmed, and
this requires either clearing the status or aborting, depending on its
current state.  Code is added to do this.  And the USB transfer size is now
automatically detected so devices with a size less than 2048 bytes work
correctly.

Some Python linters don't like unconditional except clauses because they
catch SystemExit and KeyboardInterrupt, which usually is not the intended
behaviour.

There appears to be an issue on Windows with CPython >= 3.6,
sys.stdout.flush() raises an exception:

    OSError: [WinError 87] The parameter is incorrect

It works fine to just catch and ignore the error on the flush line.  Tested
on Windows 10 x64 1803 (Build 17134.228), Python 3.6.4 amd64.

Following standard practice for defining custom exceptions.

The Python documentation recommends to pass the command as a string when
using Popen(..., shell=True).  This is because "sh -c <string>" is used to
execute the command and additional arguments after the command string are
passed to the shell itself (not the executing command).

https://docs.python.org/3.5/library/subprocess.html#subprocess.Popen

The first dynamic qstr pool is double the size of the 'alloc' field of
the last const qstr pool. The built in const qstr pool
(mp_qstr_const_pool) has a hardcoded alloc size of 10, meaning that the
first dynamic pool is allocated space for 20 entries. The alloc size
must be less than or equal to the actual number of qstrs in the pool
(the 'len' field) to ensure that the first dynamically created qstr
triggers the creation of a new pool.

When modules are frozen a second const pool is created (generally
mp_qstr_frozen_const_pool) and linked to the built in pool. However,
this second const pool had its 'alloc' field set to the number of qstrs
in the pool. When freezing a large quantity of modules this can result
in thousands of qstrs being in the pool. This means that the first
dynamically created qstr results in a massive allocation. This commit
sets the alloc size of the frozen qstr pool to 10 or less (if the number
of qstrs in the pool is less than 10). The result of this is that the
allocation behaviour when a dynamic qstr is created is identical with an
without frozen code.

Note that there is the potential for a slight memory inefficiency if the
frozen modules have less than 10 qstrs, as the first few dynamic
allocations will have quite a large overhead, but the geometric growth
soon deals with this.

This patch will work for both Python 2 and 3.

pyusb v1.0.2 warns about `getargspec` as being deprecated.

Thanks to @dhylands for this patch to pad the image to 8-byte boundaries.

Segments are separated by / and begin with the memory address.  This
follows how the ST DFU tool works.

The ST DFU bootloader supports a transfer size up to 2048 bytes, so send
that much data on each download (to device) packet.  This almost halves
total download time.

Uses new pypi.org URL, and now creates a socket with the address parameters
returned by getaddrinfo().

Way to reproduce a UnicodeEncodeError without this patch:

    LC_ALL=C tinytest-codegen.py ...

Instead of passing thru more and more options from tinytest-codegen to
run-tests --list-tests, pipe output of run-tests --list-tests into
tinytest-codegen.

This call is required before using the device (some operating systems don't
need it but others do).  Fixes issue #3476.

Gets passed to run-tests --list-tests to get actual list of tests to use.
If --target= is not given, legacy set hardcoded in tinytest-codegen itself
is used.

Also, get rid of tinytest test groups - they aren't really used for
anything, and only complicate processing. Besides, one of the next
step is to limit number of tests per a generated file to control
the binary size, which also will require "flat" list of tests.

So it was manageable and extensible.