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_const_eval/src/interpret/validity.rs

@@ -616,7 +616,7 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValidityVisitor<'rt, 'tcx, M> {
                 self.check_safe_pointer(value, PointerKind::Ref(*mutbl))?;
                 Ok(true)
             }
-            ty::FnPtr(_sig) => {
+            ty::FnPtr(..) => {
                 let value = self.read_scalar(value, ExpectedKind::FnPtr)?;
 
                 // If we check references recursively, also check that this points to a function.