at the start of the structure, making it easier to change the layout in
the future.
Also clean up the growth of various fields and consolidate some of the
metadata into a pointer to the `struct objc_method`.
Imported selector table code frm the Étoilé runtime. We can now make dispatch type dependent with a -D switch. Not enabled yet, but it will be enabled in a warning mode soon - I consider preferable to the existing GNU and Apple solution of corrupting the stack.