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_trait_selection/src/solve/trait_goals.rs

@@ -343,9 +343,10 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                     // Substitute just the unsizing params from B into A. The type after
                     // this substitution must be equal to B. This is so we don't unsize
                     // unrelated type parameters.
-                    let new_a_substs = tcx.mk_substs(a_substs.iter().enumerate().map(|(i, a)| {
-                        if unsizing_params.contains(i as u32) { b_substs[i] } else { a }
-                    }));
+                    let new_a_substs =
+                        tcx.mk_substs_from_iter(a_substs.iter().enumerate().map(|(i, a)| {
+                            if unsizing_params.contains(i as u32) { b_substs[i] } else { a }
+                        }));
                     let unsized_a_ty = tcx.mk_adt(a_def, new_a_substs);
 
                     // Finally, we require that `TailA: Unsize<TailB>` for the tail field
@@ -368,7 +369,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                     let b_last_ty = b_tys.last().unwrap();
 
                     // Substitute just the tail field of B., and require that they're equal.
-                    let unsized_a_ty = tcx.mk_tup(a_rest_tys.iter().chain([b_last_ty]).copied());
+                    let unsized_a_ty =
+                        tcx.mk_tup_from_iter(a_rest_tys.iter().chain([b_last_ty]).copied());
                     let mut nested_goals = ecx.eq(goal.param_env, unsized_a_ty, b_ty)?;
 
                     // Similar to ADTs, require that the rest of the fields are equal.
@@ -425,7 +427,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                             .map(ty::ExistentialPredicate::AutoTrait)
                             .map(ty::Binder::dummy),
                     );
-                let new_a_data = tcx.mk_poly_existential_predicates(new_a_data);
+                let new_a_data = tcx.mk_poly_existential_predicates_from_iter(new_a_data);
                 let new_a_ty = tcx.mk_dynamic(new_a_data, b_region, ty::Dyn);
 
                 // We also require that A's lifetime outlives B's lifetime.