New unicode escapes

Hello, and thank you for maintaining rust-peg!

Here are two patches to help update rust-peg for the latest version of Rust:

1. Glob imports now require an explicit `allow`, and so the first patch updates the generated code accordingly.
2. The second patch adds support for new-style "\u{...}" escape sequences. The existing "\u" and "\U" escapes are still supported for now, to ease the transition.

Unfortunately, there's one missing piece here: `libsyntax`'s pretty-printer still outputs old-style escape sequences. I'm going to look into that next, and see if it's easy to fix.