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/librustdoc/lib.rs

@@ -18,6 +18,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]