I.e. the expected result for above is b"foo", whereas previously we got
b"b'foo'".

This makes all tests pass again for 64bit windows builds which would
previously fail for anything printing ranges (builtin_range/unpack1)
because they were printed as range( ld, ld ).

This is done by reusing the mp_vprintf implementation for MICROPY_OBJ_REPR_D
for 64bit windows builds (both msvc and mingw-w64) since the format specifier
used for 64bit integers is also %lld, or %llu for the unsigned version.

Note these specifiers used to be fetched from inttypes.h, which is the
C99 way of working with printf/scanf in a portable way, but mingw-w64
wants to be backwards compatible with older MS C runtimes and uses
the non-portable %I64i instead of %lld in inttypes.h, so remove the use
of said header again in mpconfig.h and define the specifiers manually.

This uses the newly factored machine_mem functions.

Changes USB clock from 50MHz to 48MHz which improves USB communication.

Appveyor is like Travis, but for Windows builds. The appveyor.yml configuration
will build the msvc port in all configuration/platform conbinations,
and run the tests for each of those.

Requires addition of software implementation of sqrtf function.

This fixes the build after adding directory iteration in d874702f

This basically introduces the MICROPY_MACHINE_MEM_GET_READ_ADDR
and MICROPY_MACHINE_MEM_GET_WRITE_ADDR macros. If one of them is
not defined, then a default identity function is provided.

Ideally we'd use %zu for size_t args, but that's unlikely to be supported
by all runtimes, and we would then need to implement it in mp_printf.
So simplest and most portable option is to use %u and cast the argument
to uint(=unsigned int).

Note: reason for the change is that UINT_FMT can be %llu (size suitable
for mp_uint_t) which is wider than size_t and prints incorrect results.

MICROPY_ENABLE_COMPILER can be used to enable/disable the entire compiler,
which is useful when only loading of pre-compiled bytecode is supported.
It is enabled by default.

MICROPY_PY_BUILTINS_EVAL_EXEC controls support of eval and exec builtin
functions.  By default they are only included if MICROPY_ENABLE_COMPILER
is enabled.

Disabling both options saves about 40k of code size on 32-bit x86.

Saves 88 bytes on Thumb2, and 200 bytes on x86-64 archs.

To let unix port implement "machine" functionality on Python level, and
keep consistent naming in other ports (baremetal ports will use magic
module "symlinking" to still load it on "import machine").

Fixes #1701.

Compiles with mingw32, tested to work erratically under Wine due to
not fully implemented emulation in it.

This solves long-standing non-deterministic bug, which manifested itself
on x86 32-bit (at least of reported cases) - segfault on Ctrl+C (i.e.
SIGINT).

For builds where mp_uint_t is larger than size_t, it doesn't make
sense to use such a wide type for qstrs.  There can only be as many
qstrs as there is address space on the machine, so size_t is the correct
type to use.

Saves about 3000 bytes of code size when building unix/ port with
MICROPY_OBJ_REPR_D.

Only enabled for MICROPY_OBJ_REPR_D.

size_t is the correct type to use to count things related to the size of
the address space.  Using size_t (instead of mp_uint_t) is important for
the efficiency of ports that configure mp_uint_t to larger than the
machine word size.

The GC should search for pointers within the heap.  This patch makes a
difference when an object is larger than a pointer (eg 64-bit NaN
boxing).

Ports will need to #define _DIRENT_HAVE_D_INO (0) to disable d_ino use.

Similar to recently added feature in unix port: if event triggers for an
objects, its polling flags are automatically reset, so it won't be polled
until they are set again explicitly.

This includes SPI4, SPI5 and SPI6.