Added box_syntax feature gate; added to std and rustc crates for bootstrap.

To avoid using the feauture, change uses of `box <expr>` to
`Box::new(<expr>)` alternative, as noted by the feature gate message.

(Note that box patterns have no analogous trivial replacement, at
least not in general; you need to revise the code to do a partial
match, deref, and then the rest of the match.)

[breaking-change]

src/libregex/lib.rs

@@ -24,6 +24,7 @@
 
 #![allow(unknown_features)]
 #![feature(slicing_syntax)]
+#![feature(box_syntax)]
 #![deny(missing_docs)]
 
 #[cfg(test)]