Make the unstable StrExt and SliceExt traits private to libcore in not(stage0)

`Float` still needs to be public for libcore unit tests.

src/liballoc/lib.rs

@@ -75,7 +75,7 @@
 #![deny(missing_debug_implementations)]
 
 #![cfg_attr(test, allow(deprecated))] // rand
-#![cfg_attr(not(test), feature(core_float))]
+#![cfg_attr(all(not(test), stage0), feature(float_internals))]
 #![cfg_attr(not(test), feature(exact_size_is_empty))]
 #![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(test, feature(rand, test))]