Addresses issue #1390.

Otherwise it can be very hard to track down bugs.

Addresses issue #1380.

This allows a user-specified Timer for the triggering of the ADC read,
mirroring the new behaviour of DAC.write_timed.

Addresses issue #1129.

This allows the DAC to use a user-specified Timer for the triggering
(instead of the default Timer(6)), while still supporting original
behaviour.

Addresses issues #1129 and #1388.

This prevented pin_find_af* functions from being able to find some
of the alternate functions in the pin struct

Also adds #if guards to allow uPy core to compile without memoryview
enabled, but with slice assignment enabled.

Adds ability to do "memcpy" with memoryview objects, such as:
m1[0:3] = m2[2:5].

Only those files which are needed by the stmhal port are added.

Also includes a dummy file (stm32f2xx_hal_pcd_ex.c) to keep the build
system the same for f4 and f2 MCU series.

This is in preparation for supporting other MCU series, such as
STM32F2xx.  Directory structure for the HAL is now hal/f4/{inc,src},
where "f4" will in the future be different for other series.

HAL source/header files that are not use are removed to reduce the size
of the code.

This patch makes configurable, via MICROPY_QSTR_BYTES_IN_HASH, the
number of bytes used for a qstr hash.  It was originally fixed at 2
bytes, and now defaults to 2 bytes.  Setting it to 1 byte will save
ROM and RAM at a small expense of hash collisions.

TODO: Contributed code in modesp incorrectly uses private gc_* API.

Simplelink's socket send checks for the size of the packet and sends
it in chunks if the size is too large.

Previous to this patch all interned strings lived in their own malloc'd
chunk.  On average this wastes N/2 bytes per interned string, where N is
the number-of-bytes for a quanta of the memory allocator (16 bytes on 32
bit archs).

With this patch interned strings are concatenated into the same malloc'd
chunk when possible.  Such chunks are enlarged inplace when possible,
and shrunk to fit when a new chunk is needed.

RAM savings with this patch are highly varied, but should always show an
improvement (unless only 3 or 4 strings are interned).  New version
typically uses about 70% of previous memory for the qstr data, and can
lead to savings of around 10% of total memory footprint of a running
script.

Costs about 120 bytes code size on Thumb2 archs (depends on how many
calls to gc_realloc are made).

inet_pton supports both ipv4 and ipv6 addresses. Interface is also extensible
for other address families, but underlying libc inet_pton() function isn't
really extensible (e.g., it doesn't return length of binary address, i.e. it's
really hardcoded to AF_INET and AF_INET6). But anyway, on Python side, we could
extend it to support other addresses.

Required to implement UDP servers.

sendto() turns out to be mandatory function to work with UDP. It may seem
that connect(addr) + send() would achieve the same effect, but what connect()
appears to do is to set source address filter on a socket to its argument.
Then everything falls apart: socket sends to a broad-/multi-cast address,
but reply is sent from real peer address, which doesn't match filter set
by connect(), so local socket never sees a reply.

Treating it as signed lead to buffer overflow for ports >= 32768.

This usually allows to get just a single address entry.