Initial stab at lowering mutable and exterior into the type system.

src/boot/fe/parser.ml

@@ -181,13 +181,11 @@ let err (str:string) (ps:pstate) =
 
 let (slot_nil:Ast.slot) =
   { Ast.slot_mode = Ast.MODE_interior;
-    Ast.slot_mutable = false;
     Ast.slot_ty = Some Ast.TY_nil }
 ;;
 
 let (slot_auto:Ast.slot) =
   { Ast.slot_mode = Ast.MODE_interior;
-    Ast.slot_mutable = true;
     Ast.slot_ty = None }
 ;;