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Only step=1 and non-negative indexes are supported so far.

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).