Initial commit for rust mojom parser

This is the first commit (on the main branch) for our rust bindings
generator. It consists of a simple, generic parser for mojom messages.
To be generic, the parser operates on abstract mojom types and values,
rather than creating specialized code for each type in each mojom file.

The code is still very much in development, but is capable of parsing
mojom messages with header version <2 and only int/struct fields. There
are many places that need improvement before release, which are marked
by FOR_RELEASE comments.

Currently there's no way to run or test this code, tests will follow but we're prioritizing getting some code into the main branch as soon as we can.

Bug: 444509367
Change-Id: Ic36dbe70248fc6352cb6eb591b59f998d681582c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7023569
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>
Cr-Commit-Position: refs/heads/main@{#1528273}