This better disambiguates with the msg_generated.ts module, which in JS
we call "fbs", but would be better called "msg".

This is a large API refactor of deno.h which replaces
deno_send() and deno_set_response() with deno_respond().
It also adds a req_id parameter to the deno_recv_cb.

Make writeFile/writeFileSync use it.

And send() -> sendSync()

Refactors handlers.rs

The idea is that all Deno "ops" (aka bindings) should map onto
a Rust Future. By setting the "sync" flag in the Base message
users can determine if the future is executed immediately or put
on the event loop.

In the case of async futures, a promise is automatically created.
Errors are automatically forwarded and raised.

TODO:

- The file system ops in src/handler.rs are not using the thread pool
  yet. This will be done in the future using tokio_threadpool::blocking.
  That is, if you try to call them asynchronously, you will get a promise
  and it will act asynchronous, but currently it will be blocking.
- Handlers in src/handler.rs returned boxed futures. This was to make
  it easy while developing. We should try to remove this allocation.

Also removes assignCmdId as it's currently unused.