Make Regress available for Builder test

Without this line, the tests fail when run only with the test:main suite.
Implement CallbackBuilder#mapping_method_definition

This method generates a method for mapping callback arguments from FFI
types to Ruby types before calling the callback proc.
Handle mapping for callbacks with no arguments and no return value
Handle closure arguments in generated callback argument mapper
Set up mapping method in callback classes
Use generated callback mapper to map callback arguments and return value
Remove obsolete callback argument mapping code
Add tests for callback arguments of type GError

Due to the change in callback argument handling, this and many other types
are now supported.
Add several passing integration tests for Regress
Avoid unused variable warning for unused lenght parameters
Add test for Regress#test_gvariant_asv