I started to look into changing the callbacks in macroexp.h to use
gdb::function_view. However, I noticed that the passed lookup function
was always `standard_macro_lookup`, which looks up a macro in a
`macro_scope` object. Since that doesn't look like a very useful
abstraction, it would be simpler to just pass the scope around and have
the various functions call standard_macro_lookup themselves. This is
what this patch does.