constify (DoubleEnded)Iterator for RepeatN

[Lars-Schumann]

Tracking issue: #92476

constifies Iterator and DoubleEndedIterator for RepeatN.

Along the way I had to make ExactSizeIterator const, and de-non_const advance_by, nth for Iterator, and advance_back_by, nth_back for DoubleEndedIterator.

Unsure how to assert_eq! in a const compatible way, so its a panic as a placeholder.

[rustbot]

r? @scottmcm

rustbot has assigned @scottmcm.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @scottmcm, libs
• @scottmcm, libs expanded to 8 candidates
• Random selection from Mark-Simulacrum, scottmcm