https://github.com/denoland/deno_install

1. This allows the installer to use a more aggressive CI build matrix
   with different version of python.

2. Running the installer during tools/test.py artificially inflates our
   download stats.

3. Running the installer during tools/test.py makes the test script
   dependent on having an internet connection.

4. Running the installer during tools/test.py introduces a race
   condition during release - where it tries and fails to download the
   latest release in build process that should be uploading that release.

In collaboration with Tommy Savaria <tommy.savaria@protonmail.ch>

- Add build scripts for Rust flatbuffers.
- Rewrites some reply.cc methods in Rust.
- Changes some struct elements to table in msg.fbs (rust flatbuffers
  lacks support currently)
- Renames handlers_test to test_rs.
- This reorg is needed to make progress on the code cache handler.