Add peekable_map_while and peekable_take_while functions and associated structs

[devins2518]

These iterators allow you to use the map_while and take_while functions without needing to consume self. While implementing these, I discovered that both MapWhile and TakeWhile have custom implementations of fold and try_fold which I did not implement for PeekableMapWhile and PeekableTakeWhile as I didn't understand their purpose.

Any insights would be appreciated. Thank you.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

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[the8472]

The default implementations for fold and try_fold are usually overridden for performance reasons

The names could use some improvement since it repeats itself peekable() and then peekable_take_while() and it's also quite long. Maybe peek_while? No idea for peekable_map_while though.

I'm wondering whether we need this API at all in the long run. If specialization were closer to stabilization then we could simply do peekable().by_ref().map_while() and specialize that implementation to keep the peeked value when the predicate returns false, then one can drop the MapWhile and get back access to the Peekable. Although that would be a breaking change in behavior, so probably not.

[devins2518]

I've just realized that Itertools provides a similar iterator as PeekingTakeWhile. Should this PR be closed in favor of using that until specialization lands?

[the8472]

The specialization idea wasn't that great so I don't think we should really wait for that. But if itertools already has it then that may be good enough, it does seem kind of niche.