Rename many interner functions.

(This is a large commit. The changes to
`compiler/rustc_middle/src/ty/context.rs` are the most important ones.)

The current naming scheme is a mess, with a mix of `_intern_`, `intern_`
and `mk_` prefixes, with little consistency. In particular, in many
cases it's easy to use an iterator interner when a (preferable) slice
interner is available.

The guiding principles of the new naming system:
- No `_intern_` prefixes.
- The `intern_` prefix is for internal operations.
- The `mk_` prefix is for external operations.
- For cases where there is a slice interner and an iterator interner,
  the former is `mk_foo` and the latter is `mk_foo_from_iter`.

Also, `slice_interners!` and `direct_interners!` can now be `pub` or
non-`pub`, which helps enforce the internal/external operations
division.

It's not perfect, but I think it's a clear improvement.

The following lists show everything that was renamed.

slice_interners
- const_list
  - mk_const_list -> mk_const_list_from_iter
  - intern_const_list -> mk_const_list
- substs
  - mk_substs -> mk_substs_from_iter
  - intern_substs -> mk_substs
  - check_substs -> check_and_mk_substs (this is a weird one)
- canonical_var_infos
  - intern_canonical_var_infos -> mk_canonical_var_infos
- poly_existential_predicates
  - mk_poly_existential_predicates -> mk_poly_existential_predicates_from_iter
  - intern_poly_existential_predicates -> mk_poly_existential_predicates
  - _intern_poly_existential_predicates -> intern_poly_existential_predicates
- predicates
  - mk_predicates -> mk_predicates_from_iter
  - intern_predicates -> mk_predicates
  - _intern_predicates -> intern_predicates
- projs
  - intern_projs -> mk_projs
- place_elems
  - mk_place_elems -> mk_place_elems_from_iter
  - intern_place_elems -> mk_place_elems
- bound_variable_kinds
  - mk_bound_variable_kinds -> mk_bound_variable_kinds_from_iter
  - intern_bound_variable_kinds -> mk_bound_variable_kinds

direct_interners
- region
  - intern_region (unchanged)
- const
  - mk_const_internal -> intern_const
- const_allocation
  - intern_const_alloc -> mk_const_alloc
- layout
  - intern_layout -> mk_layout
- adt_def
  - intern_adt_def -> mk_adt_def_from_data (unusual case, hard to avoid)
  - alloc_adt_def(!) -> mk_adt_def
- external_constraints
  - intern_external_constraints -> mk_external_constraints

Other
- type_list
  - mk_type_list -> mk_type_list_from_iter
  - intern_type_list -> mk_type_list
- tup
  - mk_tup -> mk_tup_from_iter
  - intern_tup -> mk_tup

compiler/rustc_ty_utils/src/implied_bounds.rs

@@ -19,16 +19,16 @@ fn assumed_wf_types(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::List<Ty<'_>> {
             let mut assumed_wf_types: Vec<_> =
                 tcx.assumed_wf_types(tcx.parent(def_id)).as_slice().into();
             assumed_wf_types.extend(liberated_sig.inputs_and_output);
-            tcx.intern_type_list(&assumed_wf_types)
+            tcx.mk_type_list(&assumed_wf_types)
         }
         DefKind::Impl { .. } => {
             match tcx.impl_trait_ref(def_id) {
                 Some(trait_ref) => {
                     let types: Vec<_> = trait_ref.skip_binder().substs.types().collect();
-                    tcx.intern_type_list(&types)
+                    tcx.mk_type_list(&types)
                 }
                 // Only the impl self type
-                None => tcx.intern_type_list(&[tcx.type_of(def_id).subst_identity()]),
+                None => tcx.mk_type_list(&[tcx.type_of(def_id).subst_identity()]),
             }
         }
         DefKind::AssocConst | DefKind::AssocTy => tcx.assumed_wf_types(tcx.parent(def_id)),