Remove flatbuffers

[ry]

Depends on #2817

Fixes #2121

[bartlomieju]

Great to finally have it removed! @ry what's your idea to handle ErrorKind and MediaType? Keeping them in sync between Rust and TypeScript manually seems like a low hanging fruit for now.

[ry]

@bartlomieju Not sure yet how best to handle ErrorKind and MediaType. Maybe we need something like #[derive(TypeScript)]. I'm fine with manually keeping them in sync in the near term tho.

[rw]

I agree that this is the right move :-] Deno doesn't need most of the features FlatBuffers provides, and replacing your serialization system with an i8 enum is going to be orders of magnitude faster.

[piscisaureus]

reviewed, LGTM