Opt into new box_patterns feature gate in various crates.

Namely: `collections` (used in `dlist.rs`), `syntax`, `rustc`,
`rustc_typeck`, `rustc_trans`, and `rustdoc`.

src/libcollections/lib.rs

@@ -24,6 +24,7 @@
 
 #![feature(alloc)]
 #![feature(box_syntax)]
+#![feature(box_patterns)]
 #![feature(core)]
 #![feature(hash)]
 #![feature(staged_api)]