transmute_copy should not have alignment requirements

[arielb1]

std::mem::transmute_copy should work on unaligned pointers, unlike dereferences and ptr::read.

[Amanieu]

cc rust-lang/rfcs#1703

[newpavlov]

This issue is fixed, see the referenced issue.

[RalfJung]

It turns out indeed that transmute_copy uses read_unaligned. I had no idea. That might be worth calling out in its documentation?

Note that the reference still needs to be T-aligned, as all references do. But it does not have to be U-aligned.

[newpavlov]

It was changed in rust-lang/rust#55052.