Remove `array_vec.rs`.

Remove `array_vec.rs`.

`SparseBitSet` is the only remaining user of `ArrayVec`. This commit
switches it to using `SmallVec`, and removes `array_vec.rs`.

Why the switch? Although `SparseBitSet` is size-limited and doesn't need
the ability to spill to the heap, `SmallVec` has many more features than
`ArrayVec`. In particular, it's now possible to keep `SparseBitSet`'s
elements in sorted order, which gives in-order iteration, which is a
requirement for the next commit.