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_next_trait_solver/src/coherence.rs

@@ -334,7 +334,7 @@ where
             | ty::Str
             | ty::FnDef(..)
             | ty::Pat(..)
-            | ty::FnPtr(_)
+            | ty::FnPtr(..)
             | ty::Array(..)
             | ty::Slice(..)
             | ty::RawPtr(..)