Embedded regexes

[pczarn]

So far, only strings can be embedded.

Use the syntax r"abc.*efg" or similar. LALRPOP uses this syntax. Raw strings (r"") are equivalent to ordinary strings in Rust code, but here we give them special meaning. The downside is that using raw strings like this may seem weird. The upside is that raw strings containing backslashes work as expected, and we can pass them directly to the regular expression parser.