Add support for the Rust language.

Rust is a programming language developed by Mozilla: www.rust-lang.org
Fix Rust numbers and update keywords.
Update Rust example code for Rust 0.8.
Disambiguate RenderScript .rs files from Rust.

RenderScript isn't implemented, so if a file matches the RenderScript
detection it is just left as undetected.
Merge pull request #30 from chris-morgan/rust

Add support for Rust