Replace SliceExt with inherent [T] methods in libcore

Author: SimonSapin

src/liballoc/lib.rs

@@ -90,6 +90,7 @@
 #![feature(collections_range)]
 #![feature(const_fn)]
 #![feature(core_intrinsics)]
+#![cfg_attr(stage0, feature(core_slice_ext))]
 #![feature(custom_attribute)]
 #![feature(dropck_eyepatch)]
 #![feature(exact_size_is_empty)]