Implements "true" and "false" in regex replace. Fixes #292.

The VM is *supposed* to be doing that explicitly in `lua_tostring` (as `tostring` exhibits the proper behavior). See https://github.com/tessel/runtime/issues/393 for my looking into that. I think this is the only C code where this is exhbited, but I'll probably wind up fixing the C API to behave properly.