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].

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);