readSync and writeSync use dispatch_minimal now.

Removes the magic number hack to switch between flatbuffers and the
minimal dispatcher.

Adds machinery to pass the op_id through the shared_queue.

Due to performance regression:
https://github.com/denoland/deno/commit/dc60fe9f300043f191286ef804a365e16e455f87#commitcomment-33943711

This reverts commit dc60fe9f.

Promise id is now created in core and passed back to JS.

Fixes some sed errors introduced in c43cfe.

Unfortunately moving libdeno required splitting build.rs into two parts,
one for cli and one for core.

I've also removed the arm64 build - it's complicating things at this
re-org and we're not even testing it. I need to swing back to it and get
tools/test.py running for it.

This disables a few tests which are broken still:
- tests/error_004_missing_module.test
- tests/error_005_missing_dynamic_import.test
- tests/error_006_import_ext_failure.test
- repl_test test_set_timeout
- repl_test test_async_op
- repl_test test_set_timeout_interlaced
- all of permission_prompt_test

This is in preperation for core integration.

A new low-level crate with focus on speed. 
This doesn't yet hook into the existing code base.

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.