Feature gate *all* slice patterns. #23121

Until some backwards-compatibility hazards are fixed in #23121,
these need to be unstable.

[breaking-change]

src/librustdoc/lib.rs

@@ -39,6 +39,7 @@
 #![feature(path_ext)]
 #![feature(path_relative_from)]
 #![feature(convert)]
+#![feature(slice_patterns)]
 
 extern crate arena;
 extern crate getopts;