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

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

compiler/rustc_middle/src/traits/select.rs

@@ -125,6 +125,9 @@ pub enum SelectionCandidate<'tcx> {
     /// Builtin implementation of `DiscriminantKind`.
     DiscriminantKindCandidate,
 
+    /// Builtin implementation of `Pointee`.
+    PointeeCandidate,
+
     TraitAliasCandidate(DefId),
 
     /// Matching `dyn Trait` with a supertrait of `Trait`. The index is the