Split up NarrowingTransformer (closes #49)

Splits up NarrowingTransformer into ImmediateNarrowingTransformer and
CascadingNarrowingTransformer. All types that previously satisfied
NarrowingTransformer now satisfy ImmediateNarrowingTransformer.

Done with the following script:
https://gist.github.com/lucaswerkmeister/0bb626c381f10870906a