Feature gate *all* slice patterns. #23121

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

[breaking-change]

src/libstd/lib.rs

@@ -129,6 +129,7 @@
 #![feature(allow_internal_unstable)]
 #![feature(str_char)]
 #![feature(into_cow)]
+#![feature(slice_patterns)]
 #![cfg_attr(test, feature(test, rustc_private, std_misc))]
 
 // Don't link to std. We are std.