Deprecate `fold_while()` in favor of `Iterator::try_fold()`

[ronnodas]

This was previously discussed in #469, but it seems that was mainly about documenting an idiomatic way to rewrite an example using try_fold(). Maybe there was already a deprecation that has since been reverted? It's also mentioned in #223 as something that will be deprecated and removed.

The ControlFlow enum, which seems to have the most apt semantics, has been stable since Rust 1.55. FoldWhile does have an into_inner() method that doesn't have a direct analog on ControlFlow yet, but this seems a minor ergonomics deficiency.

[scottmcm]

Yes, if you check rust-lang/rust#75744 (comment) you'll see that it links #469 -- ControlFlow was added directly to be used for this :)

It was #476 that undeprecated it.

I'm largely peanut gallery here, but it makes sense at least to me that it would be reasonable to re-deprecate now. With the MSRV at 1.63 should even be able to use ControlFlow directly in the implementation.

itertools/Cargo.toml

Line 19 in d5897f7

rust-version = "1.63.0"