Improve the Rust lexer

[alanhdu]

This changes the Rust lexer to handle ::, :, and -> better. Previously, all of these tokens would labeled as text, which leads to poor rendering on certain styles. For instance, using a monokai HTML style w/ inline CSS, I get

where the tokens are rendered using the default (black) text which is not really visible against the background.

This changes the lexer so that:

• : and -> are considered punctuation
• :: is considered an operator -- I am not entirely sure that's the correct thing to do, but it seems like a resaonable choice
• I went through and used Text.Whitespace instead of plain Text a little more thoroughly.

[jeanas]

Thanks!