As required for further elaboration of uasyncio, like supporting baremetal
systems with wraparound timesources. This is not intended to be public
interface, and likely will be further refactored in the future.
This allows the mp_obj_t type to be configured to something other than a
pointer-sized primitive type.
This patch also includes additional changes to allow the code to compile
when sizeof(mp_uint_t) != sizeof(void*), such as using size_t instead of
mp_uint_t, and various casts.
