Don't generate glob imports in grammars

Glob imports require the user to use #[features(globs)], so let's not
generate them if we don't need to.
Allow \u{...} to be used in grammars

Note that this only handles the input side, not the output side.  Fixing
the output side will require changes to libsyntax.  Support for old
style \u and \U escapes has been left in for now to ease the transition.