Auto merge of #130887 - Soveu:repeatn, r=scottmcm

Safer implementation of RepeatN

I've seen the "Use MaybeUninit for RepeatN" commit while reading This Week In Rust and immediately thought about something I've written some time ago - https://github.com/Soveu/repeat_finite/blob/master/src/lib.rs.

Using the fact, that `Option` will find niche in `(T, NonZeroUsize)`, we can construct something that has the same size as `(T, usize)` while completely getting rid of `MaybeUninit`.
This leaves only `unsafe` on `TrustedLen`, which is pretty neat.

Author: bors