rustc_serialize: Remove unneeded feature decls.

I.e. `maybe_uninit_slice` and `new_uninit`.

Also sort the remaining features and remove an ugly, low-value comment.

compiler/rustc_serialize/src/lib.rs

@@ -1,25 +1,19 @@
 //! Support code for encoding and decoding types.
 
-/*
-Core encoding and decoding interfaces.
-*/
-
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
     html_playground_url = "https://play.rust-lang.org/",
     test(attr(allow(unused_variables), deny(warnings)))
 )]
-#![feature(never_type)]
+#![feature(allocator_api)]
 #![feature(associated_type_bounds)]
-#![feature(min_specialization)]
+#![feature(const_option)]
 #![feature(core_intrinsics)]
-#![feature(maybe_uninit_slice)]
-#![feature(new_uninit)]
-#![feature(allocator_api)]
+#![feature(inline_const)]
+#![feature(min_specialization)]
+#![feature(never_type)]
 #![feature(ptr_sub_ptr)]
 #![feature(slice_first_last_chunk)]
-#![feature(inline_const)]
-#![feature(const_option)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]
 #![deny(rustc::untranslatable_diagnostic)]