Similar to existing mp_hal_delay_ms/mp_hal_ticks_ms.

Otherwise some compilers (eg without optimisation) will put this read-only
data in RAM instead of ROM.

Both read and write operations support variants where either a) a single
call is made to the undelying stream implementation and returned buffer
length may be less than requested, or b) calls are repeated until requested
amount of data is collected, shorter amount is returned only in case of
EOF or error.

These operations are available from the level of C support functions to be
used by other C modules to implementations of Python methods to be used in
user-facing objects.

The rationale of these changes is to allow to write concise and robust
code to work with *blocking* streams of types prone to short reads, like
serial interfaces and sockets. Particular object types may select "exact"
vs "once" types of methods depending on their needs. E.g., for sockets,
revc() and send() methods continue to be "once", while read() and write()
thus converted to "exactly" versions.

These changes don't affect non-blocking handling, e.g. trying "exact"
method on the non-blocking socket will return as much data as available
without blocking. No data available is continued to be signaled as None
return value to read() and write().

From the point of view of CPython compatibility, this model is a cross
between its io.RawIOBase and io.BufferedIOBase abstract classes. For
blocking streams, it works as io.BufferedIOBase model (guaranteeing
lack of short reads/writes), while for non-blocking - as io.RawIOBase,
returning None in case of lack of data (instead of raising expensive
exception, as required by io.BufferedIOBase). Such a cross-behavior
should be optimal for MicroPython needs.

It now supports \n, \r and \r\n as newline separators.

Adds 56 bytes to stmhal and 80 bytes to unix x86-64.

Fixes issue #1689.

Address printed was truncated anyway and in general confusing to outsider.
A line which dumps it is still left in the source, commented, for peculiar
cases when it may be needed (e.g. when running under debugger).

'=' is pretty natural character for tail, and gives less dense picture
where it's easier to see what object types are actually there.

OSError's are now printed like:

    OSError: [Errno 1] EPERM

but only if the string corresponding to the errno is found.

Actual loading of .mpy files isn't tested.

In some compliation enviroments (e.g. mbed online compiler) with
strict standards compliance, <math.h> does not define constants such
as M_PI.  Provide fallback definitions of M_E and M_PI where needed.

If an OSError is raised with an integer argument, and that integer
corresponds to an errno, then the string for the errno is used as the
argument to the exception, instead of the integer.  Only works if
the uerrno module is enabled.

These are typical consumers of large chunks of memory, so it's useful to
see at least their number (how much memory isn't clearly shown, as the data
for these objects is allocated elsewhere).

Effect measured on esp8266 port:

Before:
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44214 ms
This machine benchmarks at 226 pystones/second
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44246 ms
This machine benchmarks at 226 pystones/second

After:
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44343ms
This machine benchmarks at 225 pystones/second
>>> pystone_lowmem.main(10000)
Pystone(1.2) time for 10000 passes = 44376ms
This machine benchmarks at 225 pystones/second

This reverts commit 6de8dbb4. The change
was incorrect (correct change would require comparing with end pointer in
each if statement in the block).

vstr_null_terminated_str is almost certainly a vstr finalization operation,
so it should add the requested NUL byte, and not try to pre-allocate more.
The previous implementation could actually allocate double of the buffer
size.

By comparing with string end pointer instead of checking for NUL byte.
Should alleviate reallocations and fragmentation a tiny bit.

Previous to this patch bignum division and modulo would temporarily
modify the RHS argument to the operation (eg x/y would modify y), but on
return the RHS would be restored to its original value.  This is not
allowed because arguments to binary operations are const, and in
particular might live in ROM.  The modification was to normalise the arg
(and then unnormalise before returning), and this patch makes it so the
normalisation is done on the fly and the arg is now accessed as read-only.

This change doesn't increase the order complexity of the operation, and
actually reduces code size.

This patch consolidates the Python logic for division/modulo to one place
within the bignum code.

When DIG_SIZE=32, a uint32_t is used to store limbs, and no normalisation
is needed because the MSB is already set, then there will be left and
right shifts (in C) by 32 of a 32-bit variable, leading to undefined
behaviour.  This patch fixes this bug.

Also do that only for the first word in a line. The idea is that when you
start up interpreter, high chance that you want to do an import. With this
patch, this can be achieved with "i<tab>".

The type is an unsigned 8-bit value, since bytes objects are exactly
that.  And it's also sensible for unicode strings to return unsigned
values when accessed in a byte-wise manner (CPython does not allow this).

While just a websocket is enough for handling terminal part of WebREPL,
handling file transfer operations requires demultiplexing and acting
upon, which is encapsulated in _webrepl class provided by this module,
which wraps a websocket object.

E.g. crashed when yielding from already stopped generators.