The MaybeUninit doesn't ever hold any uninitialized bytes, but if it _did_,
then this would have been UB! Because the write can write fewer than 8
bytes, it would leave the remaining bytes uninitialized.

Since the `MaybeUninit<u64>` is never uninitialized, we can remove
the use of `MaybeUninit`, and just use `u64`. Then it becomes clear
that the only thing for the unsafe block is the `copy_nonoverlapping`.

…u64_bytes.

The alternative would be to actually convert the `debug_assert` to
an `assert`, or clamp to 8, but both of these could conceivably
have a performance impact, so in the interest of being a pure
win, with no downsides, that change is left to a future editor.

I hear git has an `absorb` extension, but I don't have it. :(