exercise idea: fizzbuzz as an iterator adaptor

[coriolinus]

Implemented a little toy tonight on a whim: fizzbuzz as an iterator adaptor. After building it, it occurred to me that this may be a useful track-specific exercise at approx. the difficulty 7 level: doing it flexibly and properly takes a fair degree of familiarity with iterator adaptors, generics and trait bounds.

I'm creating this as an issue in order to gauge interest from the other maintainers. Questions I'd be interested to know the answers to:

1. Does this seem like a good exercise to add?
2. Do we need substantially more tests than this?
3. Should we retain both wrap and apply constructors, or simplify to only one, discarding the appropriate tests?
4. Does my estimate of 7 for the difficulty make sense, or is this easier/harder?

[coriolinus]

Also: ideas for a narrative? That appears to be an exercism priority at the moment.

[coriolinus]

Also again: should I discard both wrap and apply and rewrite in terms of something which can be passed to map?