lex raw string literals, like r#"blah"#

Raw string literals are lexed into regular string literals. This is okay
for them to "work" and be usable/testable, but the pretty-printer does
not know about them yet and will just emit regular string literals.
add vim syntax highlighting support for raw string literals
add token::LIT_STR_RAW(ident, num of # symbols)

Treat it as a synonym for LIT_STR for now.
pretty-print strings as the shortest of escaped or raw string literal
document raw string literals in tutorial.md and rust.md