Use lifetime elision in libcore

[Manishearth]

Makes things more greppable.

This change was made automatically by clippy + bash scripts, though I will run test passes, please review carefully.

[rust-highfive]

r? @alexcrichton

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[bluss]

I agree the loss of lifetime params where we return structs is unfortunate ((&self) -> Iter<T>), but I think the elision rules force our hand there. This is a win for consistency in both code and documentation.

r=me, but maybe you want to squash the two latter commits together, or give them less WIP-sounding commit logs?

[Manishearth]

Squashed, they were split for reviewability.

@eddyb wanted me to wait on this until we can reach consensus on what is and isn't okay to elide.

[alexcrichton]

Currently the standard library and collections and such all pretty aggressively use lifetime elision, and to the best of my knowledge these are all just signatures that haven't been updated since after lifetime elision was added. No new API written uses an explicit lifetime unless necessary, so I agree with @bluss that this is more consistent.

@bors: r+ a520568