This patch fixes two main things:
- dicts can be printed directly using '%s' % dict
- %-formatting should not crash when passed a non-dict to, eg, '%(foo)s'

Updated modbuiltin.c to add conditional support for 3-arg calls to
pow() using MICROPY_PY_BUILTINS_POW3 config parameter. Added support in
objint_mpz.c for for optimised implementation.

0j to the power of negative now raises ZeroDivisionError, and 0j to the
power of positive returns 0.

E.g. uio.BytesIO(100) will allocate buffer with 100 bytes of space.

This PR is to address issue #2812.

This patch brings the _thread module to stmhal/pyboard.  There is a very
simple round-robin thread scheduler, which is disabled if there is only
one thread (for efficiency when threading is not used).

The scheduler currently switches threads at a rate of 250Hz using the
systick timer and the pend-SV interrupt.

The GIL is disabled so one must be careful to use lock objects to prevent
concurrent access of objects.

The threading is disabled by default, one can enabled it with the config
option MICROPY_PY_THREAD to test it out.

Using MICROPY_FATFS_EXFAT.  Enabling this has licensing implications;
see https://www.microsoft.com/en-us/legal/intellectualproperty/mtl/exfat-licensing.aspx

If the SD card is mounted then its libraries (ie those that are imported)
should override any in /flash.

The first partition is mounted as "/sd" and subsequent partitions are
mounted as "/sd<part_num>".  This is backwards compatible with the previous
behaviour, which just mounted the first partition on "/sd".

At this point, only FatFs filesystems are mounted.

Also works for non-networked builds (like minimal).

The stack pointer should start pointing 1 byte past the top of the end of
RAM.

stmhal has MULTI_PARTITION enabled for FatFs and so these values need to be
initialised.

vfs.h is for generic VFS declarations, and vfs_fat.h is for VfsFat
specific things.