Add methods Iterator::find_or_{first,nth,last}

[z33ky]

I'm proposing to add the methods find_or_{first,nth,last} to the Iterator trait, to facilitate returning an item that is part of the sequence itself in case no item satisfies the predicate.

Example:

let a = [0, 1, 2, 3];

assert_eq!(a.iter().by_ref().copied().find_or_first(|e| *e == 5   ), Some(0)); // predicate is never satisfied, so `find_or_first` returns the first item (`0`)
assert_eq!(a.iter().by_ref().copied().find_or_nth(  |e| *e == 5, 1), Some(1)); // predicate is never satisfied, so `find_or_nth` returns the 2nd item (`1`) (count begins from `0`)
assert_eq!(a.iter().by_ref().copied().find_or_last( |e| *e == 5   ), Some(3)); // predicate is never satisfied, so `find_or_last` returns the last item (`3`)

Rendered

[z33ky]

I hope this isn't too terse, but that's literally all that'd happen I think.

[sfackler]

Small API additions do not require RFCs. You can just make a PR against rust-lang/rust with the proposed methods.

[z33ky]

Thanks, I opened rust-lang/rust#79271.