import works for simple cases.  Still work to do on finding the right
script, and setting globals/locals correctly when running an imported
function.

Generator objects now allocate the object and the state in one malloc.
This improvement fixes Issue #38.

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)).

This fixes Issue #29, and means the core is no longer dependent on
string functions, except strlen.

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).