No need to use `boxfnonce`

[ferjm]

<nox> Manishearth, ferjm: IMO we don't need boxfnonce, I did something similar for tasks in the script crate and it helped me quite a bit to make my own trait with a name specific to my domain (i.e. tasks).
<nox> Manishearth, ferjm: I.e. https://github.com/servo/media/blob/14de1191e3ee075c1295f8cc468208a815be8eb1/audio/src/node.rs#L192 this could be a trait which convey more information about what it is, with an impl for Box<F> where F: FnOnce.
<nox> Note that for the script tasks, I ended up having two traits anyway, TaskOnce and TaskBox, because if you try to compose the boxed ones directly you end up with multiple layers of boxing.