Replace Repr with the new ty::layout in rustc.

Lvalues still need work (see lvalue_layout).

src/lib.rs

@@ -8,9 +8,7 @@
 )]
 
 // From rustc.
-extern crate arena;
 #[macro_use] extern crate rustc;
-extern crate rustc_data_structures;
 extern crate rustc_mir;
 extern crate syntax;