Make `Vec` yokeable

[ChayimFriedman2]

Yokeable is implemented for Vec, but it does not forward the yoking and instead just treat them as owned values that do not borrow from the cart. This is inconsistent with the implementation of Yokeable for arrays, which does forward to the items.

I don't see any inherent reason why Vec couldn't do the same. This is a breaking change, but with the current situation it is impossible (safely) to use Vec as a yokeable, while if we change it it is easily possible to restore the old behavior by deriving Yokeable on non-yoked structs.

[sffc]

It seems intuitive that the items of a Vec should be able to borrow from the cart if they are not currently able to do so.

@Manishearth

[Manishearth]

This would be acceptable, I think.

[Manishearth]

Though I think the reason we have that impl is because doing such impls on third-party crates can be annoying. But we don't care too much about Vec in ICU4X so we're not relying on it anyway. I think.