FusedIterator, ExactSizeIterator, DoubleEndedIterator

[therealbnut]

Hi,

Thanks for making this great library!

I came across this today, and it seems like it should be possible to implement FusedIterator, ExactSizeIterator, and DoubleEndedIterator for Vec's IntoIter and probably others too without too much complexity.

I was thinking ExactSizeIterator at least would be useful when using these data types to extend heap allocated structures so they can allocate the correct size up front. The other traits likely have performance or functional benefits in other situations too.

What do you think? Are they intentionally not implemented yet?

[zeenix]

What do you think? Are they intentionally not implemented yet?

Sounds good to me. I think someone just haven't added these yet so please feel free to provide a PR for these.

[therealbnut]

@zeenix I've thrown up a PR #628