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/libstd/lib.rs

@@ -283,6 +283,7 @@
 #![feature(macro_vis_matcher)]
 #![feature(needs_panic_runtime)]
 #![feature(never_type)]
+#![feature(nonnull)]
 #![feature(num_bits_bytes)]
 #![feature(old_wrapping)]
 #![feature(on_unimplemented)]
@@ -297,7 +298,6 @@
 #![feature(raw)]
 #![feature(repr_align)]
 #![feature(rustc_attrs)]
-#![feature(shared)]
 #![feature(sip_hash_13)]
 #![feature(slice_bytes)]
 #![feature(slice_concat_ext)]