Remove unused generic

[tcharding]

Turns out clippy does not warn for unused generics.

Post merge edit for when we write changlog: API breaking change.

Fix: #191

[clarkmoody]

ACK d0905b0

[tcharding]

Force push is rebase only, will still need another one once #192 merges.

[tcharding]

Rebase only, no code changes.

[apoelstra]

ACK 7bdabff successfully ran local tests; note that this is technically a breaking change

[stevenroose]

@tcharding why was that unused argument added there? very confused by this

[apoelstra]

I think this was my code, and the reason is that an early iteration of this API had a ton of generic iterators. The idea was that you could turn any iterator over bytes into one over Fes, any one over Fes into one that added a checksum, etc, etc.

But it turned out that the resulting API was super confusing and undiscoverable (and sometimes impossible to implement because of stdlib bugs (ExactSizeIterator not being implemented for a chain of exact size iterators) and/or limitations in Rust generics (cannot the type produced by map)) and didn't even provide any value.

So I switched to this API with specific iterator types that have to be called in a specific order and things are simpler.

[apoelstra]

Oh, wait. Your question is about this PR which replaces the unused generic (which I explained above) with an unused u8 parameter.

Cut/paste error? Very strange. @tcharding

Which I didn't see, and I think Clark didn't see either, because the diff looks so much like line noise..

[tcharding]

@tcharding why was that unused argument added there? very confused by this

Ha, by looking at the PR I have no idea what so ever. #222