[dfmc,c-ffi] Add objc-selector-definer.

This is similar to c-function-definer, but sets up things for
simplifying usage of the objc-dylan bridge library.

This duplicates some parts of c-function-definer and re-uses many
parts of it.
Merge pull request #674 from dylan-lang/c-ffi-changes

[dfmc,c-ffi] Add objc-selector-definer.