Add snippets from language-rust

[sunjay]

Now that there is built-in Rust support, language-rust is no longer needed. We should move the snippets from that repo into this repo so that package becomes completely unnecessary.

https://github.com/zargony/atom-language-rust/blob/master/snippets/rust.cson

[alexheretic]

Sounds sensible, I don't think the built-in rust package has any intention of adding snippets.

[sunjay]

@alexheretic I created a PR for this here: #112