Boolean Literal should not belong to `tt::Literal`

Currently we treat boolean literal as `tt::Literal` as it makes parsing `$lit:lit` matcher easily. 
However, `proc-macro2` treat boolean literal as `ident` :

https://github.com/alexcrichton/proc-macro2/blob/79c8ed793f9afe4bc491575a86256651dcf777b6/src/lib.rs#L940-L950

We should change it back to `tt::ident` for future proof of supporting proc-macro. 