regex: remove the use of ~[] & some unnecessary ~'s.

The AST doesn't need ~s everywhere, so we can save allocations this way
& the enum isn't particularly large (~4 words) nor are regexes
long (normally), so the space saved in the `Cat` vector is unlikely to
be very much.

src/libregex/lib.rs

@@ -362,7 +362,7 @@
        html_root_url = "http://static.rust-lang.org/doc/master")]
 
 #![feature(macro_rules, phase)]
-#![deny(missing_doc)]
+#![deny(missing_doc, deprecated_owned_vector)]
 
 extern crate collections;
 #[cfg(test)]