Remove the `Key` type from `JobQueue`
Remove the `Key` type from `JobQueue`

This isn't actually necessary with a bit of refactoring, and in general
it makes management of `JobQueue` simpler since there's one less type to
worry about. The one main usage of `Key` vs `Unit` was that `Key` was
`Send` to be placed in `Message`, but this was replaced by just
assigning each `Unit` an ever-increasing integer, and then `Finished`
contains the integer rather than the `Key` itself which we can map once
we get back to the main thread.

tshepang/cargo,nipunn1313/cargo,tsurai/cargo,nipunn1313/cargo,rust-lang/cargo,vojtechkral/cargo,tsurai/cargo,rust-lang/cargo,rust-lang/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,tshepang/cargo,vojtechkral/cargo,nipunn1313/cargo,eminence/cargo,tshepang/cargo,eminence/cargo,rustyhorde/cargo,vojtechkral/cargo,nipunn1313/cargo,rustyhorde/cargo,tsurai/cargo,rustyhorde/cargo,eminence/cargo,tshepang/cargo,tsurai/cargo,tsurai/cargo,vojtechkral/cargo