Clean up support for "aligning the backing store of allocations"

[RalfJung]

In rust-lang/rust#108022 I raised some concerns post-landing. Let's track here that these need to be resolved, or else we should remove the interface again. In its current form it is at the very least insufficiently documented; more likely the trait needs to be adjusted.

Cc @CraftSpider

[CraftSpider]

I'm tied whether to respond here or on the original comments - I'll respond there, but copy them over here if you would rather all discussion have one spot.

[RalfJung]

I'm fine with discussing in that PR, this issue is to make sure we don't forget creating a new PR that resolves the problems. :)