There can be multiple emitters allocated during compile (eg byte code
and native).

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

Byte code has a map from byte-code offset to source-code line number,
used to give better error messages.

With MICROPY_EMIT_X64 and MICROPY_EMIT_THUMB disabled, the respective
emitters and assemblers will not be included in the code.  This can
significantly reduce binary size for unix version.

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).