raw strings

[nikomatsakis]

We are going to want something like Rust's raw strings (r"foo" or r#"foo"#, etc). We'll probably have to extend the tokenizer to support prefixes like r"foo", too, I forget if I handled that. We may also want strings that just skip the margin and whitespace munging.

We probably also want something like print that doesn't add a newline -- but maybe that could be handled by an optional argument (print(f, newline: false).await) if we supported those.