Rust: Attempt to lex constant idents

[CosmicHorrorDev]

From the Rust style guide:

https://doc.rust-lang.org/beta/style-guide/advice.html#names

• Constants (consts and immutable statics) shall be SCREAMING_SNAKE_CASE.

While giving that casing to nothing else. This means that we can attempt to lex constant idents by detecting if they are SCREAMING_SNAKE_CASE. I opted for at least 2 uppercase letters since I have seen generics use names like T0 and T1 (albeit rarely). The word boundaries (\b) are used to avoid lexing parts of idents that just so happen to have at least two consecutive uppercase letters like BTreeSet

[alecthomas]

Thanks again!