Reenable feature(nll) in alloc.

Author: m-ou-se

library/alloc/src/lib.rs

@@ -146,6 +146,7 @@
 #![feature(min_specialization)]
 #![feature(negative_impls)]
 #![feature(never_type)]
+#![feature(nll)] // Not necessary, but here to test the `nll` feature.
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
 #![feature(staged_api)]