move Option::as_(mut_)slice to use offset_of!, part 1

[llogiq]

This does the bootstrap dance to use the newly-added enum extension of the core::mem::offset_of! macro. Once that reaches beta, we can remove the bootstrap and the intrinsic.

r? @scottmcm

[the8472]

Seems redundant with #117525

[llogiq]

It's basically a stepping stone, ensuring both implementations work. But I'm ok with waiting for it to unblock.

[GKFX]

@llogiq I think this two-part approach is no longer required, as after the current beta was created #117525 is able to remove the intrinsic directly.