rename String => AbstractString
Bytes: one approach to a jl_bytes_t type.
Bytes: a new approach with less built-in stuff.
wip: more functionality for the Bytes type
wip: Str (Bytes-based string type)
Bytes: enable GC marking of Bytes objects.
Bytes: make bytesref a builtin (prep for adding it to codegen).
wip: take a crack at native getindex(::Bytes, ::Int); no dice.

This was the best I could come up with but the code is too complex
for LLVM to figure out what I'm trying to do and do it efficiently.
It seems like we really may need to provide custom code gen.