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/util/type_name.rs

@@ -35,7 +35,7 @@ impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
             | ty::Slice(_)
             | ty::RawPtr(_, _)
             | ty::Ref(_, _, _)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Never
             | ty::Tuple(_)
             | ty::Dynamic(_, _, _) => self.pretty_print_type(ty),