Consider renaming IndexedParallelIterator::collect_into

[cuviper]

rust-lang/rust#45840 is proposing an Iterator::collect_into for any Extend type, different than our IndexedParallelIterator::collect_into that truncates and collects into a Vec. I think Iterator::unzip_into is also something that could reasonably added for symmetry.

It would be unfortunate to have this mismatch long-term, but it's too early to know if std will really add this, or what shape it will take. Maybe we can find a more specific "indexed" name that's less likely to ever have a conflict with std.

[nikomatsakis]

collect_into_vec?

[cuviper]

Fixed in #521.