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/stack.rs

@@ -483,7 +483,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 | ty::Bool
                 | ty::Float(_)
                 | ty::FnDef(..)
-                | ty::FnPtr(_)
+                | ty::FnPtr(..)
                 | ty::RawPtr(..)
                 | ty::Char
                 | ty::Ref(..)