Folding revamp.

This commit makes type folding more like the way chalk does it.

Currently, `TypeFoldable` has `fold_with` and `super_fold_with` methods.
- `fold_with` is the standard entry point, and defaults to calling
  `super_fold_with`.
- `super_fold_with` does the actual work of traversing a type.
- For a few types of interest (`Ty`, `Region`, etc.) `fold_with` instead
  calls into a `TypeFolder`, which can then call back into
  `super_fold_with`.

With the new approach, `TypeFoldable` has `fold_with` and
`TypeSuperFoldable` has `super_fold_with`.
- `fold_with` is still the standard entry point, *and* it does the
  actual work of traversing a type, for all types except types of
  interest.
- `super_fold_with` is only implemented for the types of interest.

Benefits of the new model.
- I find it easier to understand. The distinction between types of
  interest and other types is clearer, and `super_fold_with` doesn't
  exist for most types.
- With the current model is easy to get confused and implement a
  `super_fold_with` method that should be left defaulted. (Some of the
  precursor commits fixed such cases.)
- With the current model it's easy to call `super_fold_with` within
  `TypeFolder` impls where `fold_with` should be called. The new
  approach makes this mistake impossible, and this commit fixes a number
  of such cases.
- It's potentially faster, because it avoids the `fold_with` ->
  `super_fold_with` call in all cases except types of interest. A lot of
  the time the compile would inline those away, but not necessarily
  always.

compiler/rustc_traits/src/chalk/db.rs

@@ -9,7 +9,7 @@
 use rustc_middle::traits::ChalkRustInterner as RustInterner;
 use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
 use rustc_middle::ty::{
-    self, AssocItemContainer, AssocKind, EarlyBinder, Ty, TyCtxt, TypeFoldable,
+    self, AssocItemContainer, AssocKind, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
 };
 
 use rustc_ast::ast;

compiler/rustc_traits/src/chalk/lowering.rs

@@ -33,9 +33,10 @@
 
 use rustc_ast::ast;
 use rustc_middle::traits::{ChalkEnvironmentAndGoal, ChalkRustInterner as RustInterner};
-use rustc_middle::ty::fold::TypeFolder;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
-use rustc_middle::ty::{self, Binder, Region, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, Binder, Region, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_span::def_id::DefId;
 
 use chalk_ir::{FnSig, ForeignDefId};