Proper support for registering builtin modules in ROM.

Comes with some refactoring of code and renaming of files.  All modules
are now named mod*.[ch].
diff --git a/py/objmodule.h b/py/objmodule.h
new file mode 100644
index 0000000..270551d
--- /dev/null
+++ b/py/objmodule.h
@@ -0,0 +1,4 @@
+void mp_module_init(void);
+void mp_module_deinit(void);
+mp_obj_t mp_module_get(qstr module_name);
+void mp_module_register(qstr qstr, mp_obj_t module);