Support multiple Dylibs in 1 flatbuffer

[vwellsTT]

We plan to support multiple dylibs in a given flatbuffer eventually, to handle cases such as multiple architectures, etc. The exact form this would take is not exactly clear (nor necessarily trivial--can same func exist in each dylib, and get dispatched at runtime somehow?). For now, we have placeholder dylib_id field, but hardcode it to 0