[Alternative] Making sure everything is aligned correctly

[elichai]

Alternative for #233

this uses vecs/box with AlignType for the regular API.
the preallocated API is the same as in #233

[elichai]

In case anyone wonders about NonNull::<[AlignedType; 0]>::dangling() as, it is better than null, because even if you accidentally do Box::from_raw(NonNull::<[T; 0]>::dangling() as NonNull<[T]>) it will be fine because ZSTs in rust only require aligned pointers (I'm working on getting that in writing 😉 )

[apoelstra]

Closed in favor of #233