Add ptr::Pointee trait (for all types) and ptr::metadata function

RFC: https://github.com/rust-lang/rfcs/pull/2580

library/core/src/lib.rs

@@ -133,6 +133,7 @@
 #![feature(stmt_expr_attributes)]
 #![feature(str_split_as_str)]
 #![feature(str_split_inclusive_as_str)]
+#![feature(trait_alias)]
 #![feature(transparent_unions)]
 #![feature(try_blocks)]
 #![feature(unboxed_closures)]