[ffigen] ObjC example (#416)

* Add example

* Skip methods that have incomplete types

Also change how method return types are parsed

* Update bindings

* Fix overly generic method return types

* Fix tests

* Bit field test

* Fix the bug

* Fix warning wording

* Update bindings

* Remove the need for a dylib

* Update README.md

* Update examples readme

* Nits

* Add test

* Fix analysis error

* Fix more analysis errors

* Fix test