This patch adds an optional Python-stack memory region, in addition to
the C stack and the Python heap.  The Python stack is enabled by the
MICROPY_ENABLE_PYSTACK option (it's disabled by default).  And then the
memory for the Python stack is initialised via:

    mp_pystack_init(pystack_start, pystack_end);

With this feature enabled the memory for the call frame of a Python
function is allocated from this region, rather than from the C stack
or heap.  The advantage of using the Python stack is that it is very
fast to allocate/deallocate, it is deterministic, and allows better
control over how much C stack is used.