Removed the unused type parameter on WorkgroupState
Revert "Factored out code that manages the pending_quit set."

Turned out that the code was not as reusable as I had thought
it would be.

This reverts commit 97dd5256d647b3bdbf0ca6ad6cfffa817de23fff.

Conflicts:
	Control/Visitor/Parallel/Common/Workgroup.hs
Factored out the code that extracts a run outcome from a supervisor outcome.
Added a UsingHandle variant of a function for convenience.