py/modsys: Implement sys.modules.

This for example will allow people to reload modules which didn't load
successfully (e.g. due to syntax error).
diff --git a/py/objmodule.c b/py/objmodule.c
index 46c5081..1f1b7ab 100644
--- a/py/objmodule.c
+++ b/py/objmodule.c
@@ -95,7 +95,8 @@
 };
 
 mp_obj_t mp_obj_new_module(qstr module_name) {
-    mp_map_elem_t *el = mp_map_lookup(&MP_STATE_VM(mp_loaded_modules_map), MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND);
+    mp_map_t *mp_loaded_modules_map = &MP_STATE_VM(mp_loaded_modules_dict).map;
+    mp_map_elem_t *el = mp_map_lookup(mp_loaded_modules_map, MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND);
     // We could error out if module already exists, but let C extensions
     // add new members to existing modules.
     if (el->value != MP_OBJ_NULL) {
@@ -200,17 +201,18 @@
 STATIC MP_DEFINE_CONST_MAP(mp_builtin_module_map, mp_builtin_module_table);
 
 void mp_module_init(void) {
-    mp_map_init(&MP_STATE_VM(mp_loaded_modules_map), 3);
+    mp_obj_dict_init(&MP_STATE_VM(mp_loaded_modules_dict), 3);
 }
 
 void mp_module_deinit(void) {
-    mp_map_deinit(&MP_STATE_VM(mp_loaded_modules_map));
+    //mp_map_deinit(&MP_STATE_VM(mp_loaded_modules_map));
 }
 
 // returns MP_OBJ_NULL if not found
 mp_obj_t mp_module_get(qstr module_name) {
+    mp_map_t *mp_loaded_modules_map = &MP_STATE_VM(mp_loaded_modules_dict).map;
     // lookup module
-    mp_map_elem_t *el = mp_map_lookup(&MP_STATE_VM(mp_loaded_modules_map), MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP);
+    mp_map_elem_t *el = mp_map_lookup(mp_loaded_modules_map, MP_OBJ_NEW_QSTR(module_name), MP_MAP_LOOKUP);
 
     if (el == NULL) {
         // module not found, look for builtin module names
@@ -236,5 +238,6 @@
 }
 
 void mp_module_register(qstr qst, mp_obj_t module) {
-    mp_map_lookup(&MP_STATE_VM(mp_loaded_modules_map), MP_OBJ_NEW_QSTR(qst), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = module;
+    mp_map_t *mp_loaded_modules_map = &MP_STATE_VM(mp_loaded_modules_dict).map;
+    mp_map_lookup(mp_loaded_modules_map, MP_OBJ_NEW_QSTR(qst), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = module;
 }