Rename std::ptr::Shared to NonNull

`Shared` is now a deprecated `type` alias.

CC https://github.com/rust-lang/rust/issues/27730#issuecomment-352800629

src/liballoc/lib.rs

@@ -103,6 +103,7 @@
 #![feature(iter_rfold)]
 #![feature(lang_items)]
 #![feature(needs_allocator)]
+#![feature(nonnull)]
 #![feature(nonzero)]
 #![feature(offset_to)]
 #![feature(optin_builtin_traits)]
@@ -110,7 +111,6 @@
 #![feature(placement_in_syntax)]
 #![feature(placement_new_protocol)]
 #![feature(rustc_attrs)]
-#![feature(shared)]
 #![feature(slice_get_slice)]
 #![feature(slice_patterns)]
 #![feature(slice_rsplit)]