Removes the implementations that depend on the user-definable trait `Copy`.
Only fix regressions to ensure merge in 1.55: Does not modify `vec::IntoIter`.
This method on the Iterator trait is doc(hidden), and about half of
implementations were doc(hidden). This adds the attribute to the
remaining implementations.
- Initialization can use `transmute_copy` to do the bitwise copy.
- `as_slice` can use `get_unchecked` and `MaybeUninit::slice_get_ref`,
and `as_mut_slice` can do similar.
- `next` and `next_back` can use the corresponding `Range` methods.
- `Clone` doesn't need any unsafety, and we can dynamically update the
new range to get partial drops if `T::clone` panics.
