[mypyc] Add primitive type for bytearray

[JukkaL]

bytearray is no longer assignable to bytes, so add a separate primitive type for it. Add bytearray() primitive op as an example (but there are very few primitives currently). All bytearray C functions go to an optional dependency, since bytearray usages aren't very common, so we don't want to compile bytearray operations on every mypyc run.