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Auto merge of #45595 - scottmcm:iter-try-fold, r=dtolnay
Short-circuiting internal iteration with Iterator::try_fold & try_rfold These are the core methods in terms of which the other methods (`fold`, `all`, `any`, `find`, `position`, `nth`, ...) can be implemented, allowing Iterator implementors to get the full goodness of internal iteration by only overriding one method (per direction). Based off the `Try` trait, so works with both `Result` and `Option` (:tada: #42526). The `try_fold` rustdoc examples use `Option` and the `try_rfold` ones use `Result`. AKA continuing in the vein of PRs #44682 & #44856 for more of `Iterator`. New bench following the pattern from the latter of those: ``` test iter::bench_take_while_chain_ref_sum ... bench: 1,130,843 ns/iter (+/- 25,110) test iter::bench_take_while_chain_sum ... bench: 362,530 ns/iter (+/- 391) ``` I also ran the benches without the `fold` & `rfold` overrides to test their new default impls, with basically no change. I left them there, though, to take advantage of existing overrides and because `AlwaysOk` has some sub-optimality due to #43278 (which 45225 should fix). If you're wondering why there are three type parameters, see issue #45462 Thanks for @bluss for the [original IRLO thread](https://internals.rust-lang.org/t/pre-rfc-fold-ok-is-composable-internal-iteration/4434) and the rfold PR and to @cuviper for adding so many folds, [encouraging me](#45379 (comment)) to make this PR, and finding a catastrophic bug in a pre-review.
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