Tracking Issue for peekable_next_if

[jyn514]

This is a tracking issue for the feature peekable_next_if (#72310).
The feature gate for the issue is #![feature(peekable_next_if)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also uses as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should next_if be named consume_if or eat_if?
• Should next_if_eq be named consume or eat?

[jyn514]

@rustbot modify labels: T-libs

[Stupremee]

I don't think the methods should be renamed to something else, becuase Iterator also has a next method, and Peekable
should use the same name but with the if component.

Also, if these questions are resolved and there are no other blockers, I would like to go ahead and stabilize this feature.