Remove mp_obj_type_t.methods entry and use .locals_dict instead.

Originally, .methods was used for methods in a ROM class, and
locals_dict for methods in a user-created class.  That distinction is
unnecessary, and we can use locals_dict for ROM classes now that we have
ROMable maps.

This removes an entry in the bloated mp_obj_type_t struct, saving a word
for each ROM object and each RAM object.  ROM objects that have a
methods table (now a locals_dict) need an extra word in total (removed
the methods pointer (1 word), no longer need the sentinel (2 words), but
now need an mp_obj_dict_t wrapper (4 words)).  But RAM objects save a
word because they never used the methods entry.

Overall the ROM usage is down by a few hundred bytes, and RAM usage is
down 1 word per user-defined type/class.

There is less code (no need to check 2 tables), and now consistent with
the way ROM modules have their tables initialised.

Efficiency is very close to equivaluent.
diff --git a/py/builtin.c b/py/builtin.c
index 1e022be..fe9757f 100644
--- a/py/builtin.c
+++ b/py/builtin.c
@@ -152,7 +152,6 @@
     // TODO make this function more general and less of a hack
 
     mp_map_t *map = NULL;
-    const mp_method_t *meth = NULL;
     if (n_args == 0) {
         // make a list of names in the local name space
         map = rt_locals_get();
@@ -164,9 +163,6 @@
         } else if (type->locals_dict != MP_OBJ_NULL && MP_OBJ_IS_TYPE(type->locals_dict, &dict_type)) {
             map = mp_obj_dict_get_map(type->locals_dict);
         }
-        if (type->methods != NULL) {
-            meth = type->methods;
-        }
     }
 
     mp_obj_t dir = mp_obj_new_list(0, NULL);
@@ -177,11 +173,7 @@
             }
         }
     }
-    if (meth != NULL) {
-        for (; meth->name != MP_QSTR_NULL; meth++) {
-            mp_obj_list_append(dir, MP_OBJ_NEW_QSTR(meth->name));
-        }
-    }
+
     return dir;
 }