Add Iter.map_while

[OlivierNicole]

I found myself needing this function and didn’t find an easy way to make it in an efficient way by composing simpler functions.

[c-cube]

This seems quite useful indeed. I do have one design question: what if you want to stop before yielding any element (e.g. if the first element is "wrong")?

Would this be more general?

val map_while : ('a -> [`Yield of 'b | `Return of 'b | `Stop]) -> 'a t -> 'b t

where yield means, yield and continue, and return means yield and stop iterating

[OlivierNicole]

It does seem more general.

[OlivierNicole]

I implemented this version.
Is there a requirement to be compatible with OCaml 4.08? The CI fails for this reason.

[c-cube]

Yes, I'd like to preserve compatibility with 4.08 for now. You can use CCList.equal instead.

[c-cube]

I'll merge and fix the tests. Thank you :)