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_middle/src/traits/solve.rs

@@ -30,20 +30,18 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
         self,
         folder: &mut F,
     ) -> Result<Self, F::Error> {
-        Ok(FallibleTypeFolder::interner(folder).intern_external_constraints(
-            ExternalConstraintsData {
-                regions: (),
-                opaque_types: self
-                    .opaque_types
-                    .iter()
-                    .map(|opaque| opaque.try_fold_with(folder))
-                    .collect::<Result<_, F::Error>>()?,
-            },
-        ))
+        Ok(FallibleTypeFolder::interner(folder).mk_external_constraints(ExternalConstraintsData {
+            regions: (),
+            opaque_types: self
+                .opaque_types
+                .iter()
+                .map(|opaque| opaque.try_fold_with(folder))
+                .collect::<Result<_, F::Error>>()?,
+        }))
     }
 
     fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
-        TypeFolder::interner(folder).intern_external_constraints(ExternalConstraintsData {
+        TypeFolder::interner(folder).mk_external_constraints(ExternalConstraintsData {
             regions: (),
             opaque_types: self.opaque_types.iter().map(|opaque| opaque.fold_with(folder)).collect(),
         })