Rust build

Currently, the Rust components are built differently on Android vs Linux. We are missing Rust support in our GN toolchain so we currently build the Rust libraries as a staticlib and link in C++. This may change in the future once we have better support.

For now, you can build all of the Rust code using Cargo.

There are some dependencies:

• You must have the protobuf-compiler package installed
• You must have a recent version of Cargo + Rust

You should use build.py at the root to do your Rust builds so that it correctly points your dependencies towards the vendored crates and sets your $CARGO_HOME to the correct location.