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/modsys.c b/py/modsys.c
index 57ac2bf..dd4d4d5 100644
--- a/py/modsys.c
+++ b/py/modsys.c
@@ -185,6 +185,9 @@
     { MP_ROM_QSTR(MP_QSTR_stderr), MP_ROM_PTR(&mp_sys_stderr_obj) },
     #endif
 
+    #if MICROPY_PY_SYS_MODULES
+    { MP_OBJ_NEW_QSTR(MP_QSTR_modules), MP_OBJ_FROM_PTR(&MP_STATE_VM(mp_loaded_modules_dict)) },
+    #endif
     #if MICROPY_PY_SYS_EXC_INFO
     { MP_ROM_QSTR(MP_QSTR_exc_info), MP_ROM_PTR(&mp_sys_exc_info_obj) },
     #endif