Add some features which are already enabled in the unix port and
default to using the Python stack for scoped allocations: this can be
more performant in cases the heap is heavily used because for example
the memory needed for storing *args and **kwargs doesn't require
scanning the heap to find a free block.
For MSVC off_t is defined in sys/types.h but according to the comment
earlier in mpconfigport.h this cannot be included directly.
So just make off_t the same as mp_off_t.
This fixes the build for MSVC with MICROPY_STREAMS_POSIX_API
enabled because stream.h uses off_t.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
