Remove mp_obj_type_t.methods entry and use .locals_dict instead.
Originally, .methods was used for methods in a ROM class, and locals_dict for methods in a user-created class. That distinction is unnecessary, and we can use locals_dict for ROM classes now that we have ROMable maps. This removes an entry in the bloated mp_obj_type_t struct, saving a word for each ROM object and each RAM object. ROM objects that have a methods table (now a locals_dict) need an extra word in total (removed the methods pointer (1 word), no longer need the sentinel (2 words), but now need an mp_obj_dict_t wrapper (4 words)). But RAM objects save a word because they never used the methods entry. Overall the ROM usage is down by a few hundred bytes, and RAM usage is down 1 word per user-defined type/class. There is less code (no need to check 2 tables), and now consistent with the way ROM modules have their tables initialised. Efficiency is very close to equivaluent.
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