[WIP] Implement FusedIterator

[Stebalien]

Pending RFC rust-lang/rfcs#1581

Only implements FusedIterator for core.

/cc @bluss, @apasel422

[rust-highfive]

r? @aturon

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[apasel422]

CC #32586 (comment).

[Stebalien]

Unless I'm mistaken, that doesn't apply here (yet) because we don't have the lattice rule. That is, a specialized implementation would have to have the bound I: FusedIterator + Something and the implementation on FusedIterator isn't marked default. However, that's a good point. I'll make a note of this on the specialization issue.

[hexsel]

I think the RFC link is to the wrong repo, should be rust-lang/rfcs#1581

[Stebalien]

@hexsel github auto-linking... thanks.

[apasel422]

@Stebalien Here's another way this could be done: https://play.rust-lang.org/?gist=1c74c425ab7f916b691053348b27cb61&version=nightly&backtrace=0.

[bors]

☔ The latest upstream changes (presumably #33079) made this pull request unmergeable. Please resolve the merge conflicts.

[Stebalien]

@apasel422 that certainly gets rid of the unnecessary done field which is quite nice. However having all those spec_* functions is a bit unfortunate. The future of specialization of associated types looks a bit uncertain (discussion at #33090) at the moment so I'd rather leave this implementation as-is until that gets settled.

[alexcrichton]

Closing due to inactivity, but feel free to reopen once the RFC is accepted!