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/mod.rs

@@ -567,7 +567,7 @@ fn compute_external_query_constraints<'tcx>(
 ) -> Result<ExternalConstraints<'tcx>, NoSolution> {
     let region_obligations = infcx.take_registered_region_obligations();
     let opaque_types = infcx.take_opaque_types_for_query_response();
-    Ok(infcx.tcx.intern_external_constraints(ExternalConstraintsData {
+    Ok(infcx.tcx.mk_external_constraints(ExternalConstraintsData {
         // FIXME: Now that's definitely wrong :)
         //
         // Should also do the leak check here I think
@@ -616,8 +616,7 @@ pub(super) fn response_no_constraints<'tcx>(
             var_values: CanonicalVarValues::make_identity(tcx, goal.variables),
             // FIXME: maybe we should store the "no response" version in tcx, like
             // we do for tcx.types and stuff.
-            external_constraints: tcx
-                .intern_external_constraints(ExternalConstraintsData::default()),
+            external_constraints: tcx.mk_external_constraints(ExternalConstraintsData::default()),
             certainty,
         },
     })