Eg b"123" + bytearray(2) now works.  This patch actually decreases code
size while adding functionality: 32-bit unix down by 128 bytes, stmhal
down by 84 bytes.

For this, introduce MICROPY_MODULE_DICT_SIZE config setting.

This needs to be done before actually tagging the new version.

Uninitialised struct members get a default value of 0/false, so this is
not strictly needed.  But it actually decreases code size because when
all members are initialised the compiler doesn't need to insert a call
to memset to clear everything.  In other words, setting 1 extra member
to 0 uses less code than calling memset.

ROM savings in bytes: 32-bit unix: 100; bare-arm: 44; stmhal: 52.

Addresses issue #953.

This build is primarily intended for benchmarking, and may have random
features enabled/disabled to get high scores in synthetic benchmarks.
The intent is to show/prove that MicroPython codebase can compete with
CPython, when configured appropriately. But the main MicroPython aim
still remains to optimize for memory usage (which inevitibly leads to
performance degradation in some areas on some workloads).

Also restrict higher frequencies to have a VCO_OUT frequency below
432MHz, as specified in the datasheet.

Docs improved to list allowed frequencies, and explain about USB
stability.

Undefined behavior in C, needs explicit check.

gc.enable/disable are now the same as CPython: they just control whether
automatic garbage collection is enabled or not.  If disabled, you can
still allocate heap memory, and initiate a manual collection.

Addresses issue #950.

Addresses issue #934.

The inline docs (prefixed with /// in .c files) have been converted to
RST format and put in the docs subdirectory.

This is experimental support.  API is subject to changes.  RTS/CTS
available on UART(2) and UART(3) only.  Use as:

    uart = pyb.UART(2, 9600, flow=pyb.UART.RTS | pyb.UART.CTS)

This patch also enables non-blocking streams on stmhal port.

One can now make a USB-UART pass-through function:

def pass_through(usb, uart):
    while True:
        select.select([usb, uart], [], [])
        if usb.any():
            uart.write(usb.read(256))
        if uart.any():
            usb.write(uart.read(256))

pass_through(pyb.USB_VCP(), pyb.UART(1, 9600))

msvc does not treat 1L a 64bit integer hence all occurences of shifting it left or right
result in undefined behaviour since the maximum allowed shift count for 32bit ints is 31.
Forcing the correct type explicitely, stored in MPZ_LONG_1, solves this.

Original motivation is to support converting bytearrays, but easier to just
support buffer protocol at all.

It should be fair to say that almost in all cases where some API call
expects string, it should be also possible to pass byte string. For example,
it should be open/delete/rename file with name as bytestring. Note that
similar change was done quite a long ago to mp_obj_str_get_data().