Add zipping and merging components

[blamario]

A zip component would be similar to zip and zipWith functions on lists, except it would be working on two sources and sending results into a sink. It would have to stop when either of the sources is exhausted.

A merge component would also take two sources and one sink, but they'd all need to have the same type and it would run as long as either source provides more input.

It's not too clear what combinators could apply to these component types.