Refactor and fix FIXME's in mtwt hygiene code

- Moves mtwt hygiene code into its own file
- Fixes FIXME's which leads to ~2x speed gain in expansion pass
- It is now @-free

src/libsyntax/lib.rs

@@ -87,6 +87,7 @@ pub mod ext {
         pub mod macro_rules;
     }
 
+    pub mod mtwt;
 
     pub mod cfg;
     pub mod fmt;