Add quick impl for MP_BC_LOAD_CONST_BYTES which just creates qstr so far.

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)).
Merge pull request #36 from pfalcon/b-string-prefix

Add quick impl for MP_BC_LOAD_CONST_BYTES which just creates qstr so far...