Shrink TyKind::FnPtr.

By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and
`FnHeader`, which can be packed more efficiently. This reduces the size
of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms.
This reduces peak memory usage by a few percent on some benchmarks. It
also reduces cache misses and page faults similarly, though this doesn't
translate to clear cycles or wall-time improvements on CI.

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -812,7 +812,7 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                 return upvars.visit_with(self);
             }
 
-            ty::FnPtr(_) => {
+            ty::FnPtr(..) => {
                 // Let the visitor iterate into the argument/return
                 // types appearing in the fn signature.
             }