py: Change module globals from mp_map_t* to mp_obj_dict_t*.

Towards addressing issue #424.

Had a small increase to ROM usage (order 60 bytes).
diff --git a/py/modcollections.c b/py/modcollections.c
index 3401600..dbd1896 100644
--- a/py/modcollections.c
+++ b/py/modcollections.c
@@ -9,16 +9,19 @@
     { MP_OBJ_NEW_QSTR(MP_QSTR_namedtuple), (mp_obj_t)&mp_namedtuple_obj },
 };
 
-STATIC const mp_map_t mp_module_collections_globals = {
-    .all_keys_are_qstrs = 1,
-    .table_is_fixed_array = 1,
-    .used = sizeof(mp_module_collections_globals_table) / sizeof(mp_map_elem_t),
-    .alloc = sizeof(mp_module_collections_globals_table) / sizeof(mp_map_elem_t),
-    .table = (mp_map_elem_t*)mp_module_collections_globals_table,
+STATIC const mp_obj_dict_t mp_module_collections_globals = {
+    .base = {&mp_type_dict},
+    .map = {
+        .all_keys_are_qstrs = 1,
+        .table_is_fixed_array = 1,
+        .used = sizeof(mp_module_collections_globals_table) / sizeof(mp_map_elem_t),
+        .alloc = sizeof(mp_module_collections_globals_table) / sizeof(mp_map_elem_t),
+        .table = (mp_map_elem_t*)mp_module_collections_globals_table,
+    },
 };
 
 const mp_obj_module_t mp_module_collections = {
     .base = { &mp_type_module },
     .name = MP_QSTR_collections,
-    .globals = (mp_map_t*)&mp_module_collections_globals,
+    .globals = (mp_obj_dict_t*)&mp_module_collections_globals,
 };