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.
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