This is based on the fact that qstr so far behaves more like byte string
than like Unicode string (for example, "012"[0] returns 48 (int)).

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).