Implement non-crashing data conversion for Callback
Remove useless type mapping entry
Add a TODO
Merge branch 'master' into alternative-callback-logic

Conflicts:
	lib/gir_ffi/builders/callback_builder.rb
	lib/gir_ffi/callback_base.rb
	lib/gir_ffi/info_ext/i_type_info.rb
Refactor CallbackBuilder
Use alternative callback builder [merges 'alternative-callback-logic']