Remove nightly workaround for VecDeque::append being broken

[yorickpeterse]

57cc1f4 adds two workarounds for the problem described in rust-lang/rust#53529. Once the upstream issue has been resolved, the code should be changed back to using VecDeque::append.

[yorickpeterse]

For the Mailbox structure we moved from VecDeque to LinkedList, since this saves quite a bit of space in the type. Since we only ever push/pop, and the number of values will be fairly small, the performance shouldn't differ that much. For the Queue structure I'm not sure if this would be a good idea, as it's possible to have many values stored at once, and LinkedList probably adds some overhead in this case.

[yorickpeterse]

closed via commit 6990ca9