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

Adds machinery to pass the op_id through the shared_queue.

Tip: RUSTC_WRAPPER should be unset for incremental builds to work.

And throws without allow-env 

Deno.core.dispatch() used to push the "control" buf onto the shared
array buffer before calling into V8, with the idea that it was one less
argument to parse. Turns out there is no more overhead passing the
control ArrayBuffer directly over. Furthermore this optimization was
making the refactors outlined in #2730 more complex. Therefore it is
being removed.

This is a minor feature which complicates the build signifigantly.
Removing to ease refactoring the build system:
https://github.com/denoland/deno/issues/2608

A crate can assign an alternative name, different from the crate name,
when importing another crate. On the command line this looks like:

  rustc ... --extern foo_crate=path/to/bar_crate.rlib

We need to support this so we can eventually upgrade to rand-0.7.x.

based on https://w3c.github.io/FileAPI/#convert-line-endings-to-native

* remove fetch_source_file_and_maybe_compile_async and 
  replace it with State.fetch_compiled_module

* remove SourceFile.js_source()

* introduce CompiledModule which is basically the same as
  deno::SourceInfo and represents arbitrary file that has been 
  compiled to JS module

* introduce //cli/compilers module containing all compilers

* introduce JsCompiler which is a no-op compiler
  - output is the same as input, no compilation takes place
  - it is used for MediaType::JavaScript and MediaType::Unknown

* introduce JsonCompiler that wraps JSON in default export

* support JS-to-JS compilation using checkJs

* merge SourceFileFetcher trait and FileFetcher struct

* move logic related to source file fetching to //cli/file_fetcher.rs

* use Result when creating new ThreadSafeState

Symmetric with `readAll()` and `readAllSync()`. Also used in `xeval`.
Also correct usage in `writeFile()`/`writeFileSync()`.