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_middle/src/ty/context.rs

@@ -427,7 +427,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
             | ty::RawPtr(_, _)
             | ty::Ref(_, _, _)
             | ty::FnDef(_, _)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Dynamic(_, _, _)
             | ty::Closure(..)
             | ty::CoroutineClosure(..)