Mostly just a global search and replace. Except rt_is_true which
becomes mp_obj_is_true.
Still would like to tidy up some of the names, but this will do for now.
A big change. Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object). This scheme follows CPython. Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.
Also change name prefix, from py_ to mp_ (mp for Micro Python).