Implement `par_exact_chunks/_mut`; slice chunks with exact size.

[Kerollmops]

As new methods as been added to the slice type. I propose the add of these new methods too.
These iterators seems to be really useful to llvm and its optimizations.

Here is the tracking issue for exact_chunks/_mut; slice chunks with exact size.

[cuviper]

(I labelled this parity-std::iter, but that's really std::slice -- maybe we need to use more labels...)

That addition sounds fine to me, but I'd prefer to wait for it to stabilize in std before we mimic it.

[zesterer]

poke

This is stable now. It would be awesome to see Rayon gain this feature. If help is needed, I'm willing to do the leg work.

[cuviper]

Help is welcome, if this is a feature that interests you!

[sdroege]

Might also be interesting to have the rchunks_* iterators available from rayon in addition to this to have more API-equivalence between std and rayon

[cuviper]

I just merged #629 for par_chunks_exact/_mut and filed #734 for rchunks.