stream: Use mp_obj_is_true() for EOF testing.

Getting a length of string may be expensive, depending on the underlying
implementation.
Merge branch 'gc-pointers' of github.com:stinos/micropython into stinos-gc-pointers
py, gc: Revert ret_ptr to void*, casting to byte* for memset.
Merge branch 'stinos-gc-pointers'
objstr: Be 8-bit clean even for repr().

This will allow roughly the same behavior as Python3 for non-ASCII strings,
for example, print("<phrase in non-Latin script>".split()) will print list
of words, not weird hex dump (like Python2 behaves). (Of course, that it
will print list of words, if there're "words" in that phrase at all, separated
by ASCII-compatible whitespace; that surely won't apply to every human
language in existence).
unicode: String API is const byte*.

We still have that char vs byte dichotomy, but majority of string operations
now use byte.
unicode: Add trivial implementation of unichar_charlen().
objstr: Refactor to work with char pointers instead of indexes.

In preparation for unicode support.
msvc: Enable GC and use custom function for getting bss start and end