Mark several ascii methods as unstable again

We don't want to stabilize them now already. The goal of this set of
commits is just to add inherent methods to the four types. Stabilizing
all of those methods can be done later.

src/liballoc/lib.rs

@@ -83,6 +83,7 @@
 #![cfg_attr(not(test), feature(generator_trait))]
 #![cfg_attr(test, feature(rand, test))]
 #![feature(allow_internal_unstable)]
+#![feature(ascii_ctype)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]