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This is the very initial stages of writing a bindings generator to allow
interop to/from JS with FIDL.

The main pieces are:
- the build time generator which consumes the JSON IR from `fidlc` and
  outputs a fidl.js
- integration with gn in fidl_library.gni
- a JS runtime support library (mostly helps with message encoding)
- new unittest binary (which also currently holds some Zircon
  integration points that will eventually be moved to a C++ runtime
  support library.)

There are many, many things it does not yet handle, e.g. responses
(either sync or async), events, passing structs, passing unions, ...

For reference, test/simple.fidl generates this
https://gist.github.com/sgraham/3935c64d5bc1b67eea1f6e3b38fef6f1


Bug: 883496
Change-Id: I58e5d2b81213f20a3198b68cbd2bc01660ba58f7
Reviewed-on: https://chromium-review.googlesource.com/c/1222697Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596329}