[Rust] more support + fix for #41

[alexpw]

• Further fleshes out rust support
• Adds specs
• Should resolve toggle_multiline() breaks casting in Rust #41

Caveat: I don't know/use rust, but I ref'd the rust-lang book, etc.

[alexpw]

Pulled back in to draft because it's currently not handling this case correctly.

fn test() {
    let foo = stuff();
   foo != foo;
}

and produces:

fn test() { let foo = stuff();foo != foo;  }

(missing space between semi-colon and extra space before closing curly brace)

Will work on a solution to the above when I get another minute.