Fix ICE when using Iterator in const contexts

[xonx4l]

In this PR built-in candidates are implemented for Iterator and FusedIterator to resolve the issue in #151901

-> implement built-in candidates for Iterator and FusedIterator.
-> Added and blessed test for the built-in candidates implementation.

[rustbot]

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[Kivooeo]

cc @lcnr

[fee1-dead]

i appreciate the effort, but I am pretty sure this is just wrong.... we shouldn't make every type implement iterator

[fee1-dead]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[xonx4l]

@fee1-dead Sorry I was a bit busy. You're right . I was thinking too broad . I Think what if we return an error instead of a success that might work . what you say? Thanks for the feedback.

[rust-bors]

☔ The latest upstream changes (presumably #156324) made this pull request unmergeable. Please resolve the merge conflicts.