Delete or move Emitter implementations

[abhinav]

We should delete the provided emitter implementations to reduce the dependency footprint of using cff.
Custom implementations are easy to write, and requirements vary.

More importantly, this simplifies CFF's API contract:
it removes the messages we'll log and the metrics we'll emit from the contract.

Alternatively, we can move them to subpackages.
If we keep them around, we should reconsider having their constructors (ZapEmitter and TallyEmitter) return interfaces instead of structs.