Add chunk_while iterator

[pwoolcoc]

This is an iterator that is a mix of std::slice::chunks and fold.
Instead of chunking the source up into predictable lengths though, it
allows the user to inspect each element of the source, along with a
state, and decide when the chunk is big enough.

[pwoolcoc]

Sorry if I should have taken this to the stdlib first, but given

if your feature involves heap allocation, such as storing elements in a Vec, then it can't be accepted into libcore, and you should propose it for itertools directly instead.

I figured I would just bring it here, since it uses a Vec internally

[pwoolcoc]

two things I'm unhappy with:

1. type Item = Vec<T>. I wish that could be type Item = &'a [T], but then I run into the problem of tying 'a to self in fn next(&mut self, ...). If anyone has a way I can do this, I'm all ears.
2. B: Clone. I'm not sure if there's a way to do this or not, but every time it starts accumulating a new chunk, it needs to reset the state. Again, if there's a better way, I'd love to hear about it

[phimuemue]

Hi there!

Regarding type Item=Vec<T>: Do you think we could we make this a "lazy" chunking operation? (iirc such as GroupBy.)

[pwoolcoc]

@phimuemue thanks, I'm looking into that right now and I'll update here when I can

[Philippe-Cholet]

@pwoolcoc Hi, do you want to update your work on chunk_while here?

[Philippe-Cholet]

I would prompty reopen if needed.