Implement Itertools::multiunzip

[Veykril]

Simple implementation of Iterator::unzip but for tuples of sizes 0-12.

This requires adding a new public trait to be able to express the dependency between the iterator item tuple to the collection output tuple. But given that a multiunzip function is added to the Itertools trait the MultiUnzip trait does not have to be imported to make use of this.

Another option would be a macro but that would require giving it some extra syntax to declare the arity/return type which seems suboptimal, while this approach just works as a method call akin to std's iterator unzip.

Closes #362

[phimuemue]

Hi there, thanks for this! I would be willing to see this functionality in itertools.

I really like how you mimic unzip's inner workings as far as possible, and document where this is not yet possible.

[phimuemue]

lgtm.

@jswrenn If there are no objections, I'd consider merging, as this is a counterpart to the existing multizip, and it does its job - as far as I can see - reasonably well, considering the circumstances of stable rust.

[phimuemue]

Thanks for this.

bors r+

[bors]

Build succeeded:

• bors build finished