The `arrayvec::ArrayVec` type is not `Copy` which is not nice and we
would like to have a `Copy` type in our crates. While the PR to add
support to the `arrayvec` crate is not merged we implement our own
simplified version.
This one acts mostly as a dumb storage - it has just a few methods and
traits. The new ones can be added as needed later.
Martin Habovstiak