impl par_iter for other std collections. about rayon

[cuviper]

Vectors and slices work because they can be directly partitioned. How would you do this for other collection types?

[nikomatsakis]

So, some thoughts:

First, we could naturally handle hashmaps and other collections by creating (temporary) vectors and parallelizing over those. It'd add overhead but probably not that much, and we could I think do it relatively transparently.

I've also been wanting to explore bridges to/from normal sequential iterators for a while. It should be possible to handle them, even if they don't create fully balanced task trees (and hence don't necessarily scale quite as well). That was I think part of the motivation for https://github.com/nikomatsakis/rayon/pull/71; though the precise requirements are a bit out of tree for me just now.