py/runtime: Make sys.modules preallocate to a configurable size.

This allows configuring the pre-allocated size of sys.modules dict, in
order to prevent unwanted reallocations at run-time (3 sys-modules is
really not quite enough for a larger project).

py/runtime.c

@@ -91,7 +91,7 @@ void mp_init(void) {
     #endif
 
     // init global module dict
-    mp_obj_dict_init(&MP_STATE_VM(mp_loaded_modules_dict), 3);
+    mp_obj_dict_init(&MP_STATE_VM(mp_loaded_modules_dict), MICROPY_LOADED_MODULES_DICT_SIZE);
 
     // initialise the __main__ module
     mp_obj_dict_init(&MP_STATE_VM(dict_main), 1);