Replace mk_foo calls with infer_foo where possible.

There are several `mk_foo`/`intern_foo` pairs, where the former takes an
iterator and the latter takes a slice. (This naming convention is bad,
but that's a fix for another PR.)

This commit changes several `mk_foo` occurrences into `intern_foo`,
avoiding the need for some `.iter()`/`.into_iter()` calls. Affected
cases:
- mk_type_list
- mk_tup
- mk_substs
- mk_const_list

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -418,7 +418,7 @@ impl<'tcx> ir::TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::PolyExistential
 
 impl<'tcx> ir::TypeFoldable<TyCtxt<'tcx>> for &'tcx ty::List<ty::Const<'tcx>> {
     fn try_fold_with<F: FallibleTypeFolder<'tcx>>(self, folder: &mut F) -> Result<Self, F::Error> {
-        ty::util::fold_list(self, folder, |tcx, v| tcx.mk_const_list(v.iter()))
+        ty::util::fold_list(self, folder, |tcx, v| tcx.intern_const_list(v))
     }
 }