Implement Continuation.
Remove a no-longer-applicable comment.
Merge remote-tracking branch 'origin/master' into reducerof-continuations

Conflicts:
	Traversal.xcodeproj/project.pbxproj
Continuation’s tests depend on Prelude.
We can be more concise with the types if we don’t make them up as we go along.
const concision.
Merge branch 'master' into reducerof-continuations
Correct broken indentation.
Correct typo.
Better test of sequencing against ReducibleType.
Add & test a stream reducer type.
Empty inner reduction continues outer reduction.
Use the new ReducerOf implementation everywhere.
Remove Continuation.
Merge pull request #34 from robrix/reducerof-continuations

`ReducerOf` continuations