Add nested scopes spec to appendix.

Add nested scopes spec to appendix.

Add new opcodes LOAD_CLOSURE, LOAD_DEREF, STORE_DEREF, MAKE_CLOSURE to
docs for dis module.

Add docs for new function and code members in Sec. 3 of ref manual.
They're present regardless of whether nested scopes are used.

Remove description of default argument hack from Sec. 7 of the ref
manual and refer the reader to the appendix.