Improve type handling in Rust lexer
Improve type handling in Rust lexer

These changes add parsing for (and thus highlighting for) non-stdlib types and
the '&' symbol for borrows.

sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments,sol/pygments