Older int-only encoding is not expressive enough to support arbitrary slice
assignment operations.

Now of the form MICROPY_PY_*.  See issue #35.

See issue #608 for justification.

Done in generalized manner, allowing any stream class to be specified as
working with bytes.

sep=None is TODO.

This is not fully correct re: error handling, because we should check that
that types are used consistently (only str's or only bytes), but magically
makes lot of functions support bytes.

Should support everything supported by strings.

Addresses #567.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Specifically, nlr.h does.

Share code with .strip(). TODO: optimize .rstrip().

When querying an object that supports the buffer protocol, that object
must now return a typecode (as per binary.[ch]).  This does not have to
be honoured by the caller, but can be useful for determining element
size.

Also make consistent use of MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL.
This helps a lot in debugging and understanding of function API.

mp_obj_t->subscr now does load/store/delete.

Disabled by default.  Enabled in unix port.

Similar to tuples, lists, dicts. Statically allocated strings don't have hash
computed.

This feature was proposed with initial hashing RFC, and is prerequisite for
seamless static str object definition.

These largely duplicate str() & bytes() constructors' functionality,
but can be used to achieve Python2 compatibility.

It has (again) a fast path for ints, and a simplified "slow" path for
everything else.

Also simplify the way str indexing is done (now matches tuple and list).