* Improve complexity of CompactAssignments::unique_targets
Original implementation was O(n**2). Current impl is O(n log n).
Avoided the original proposed mitigation because it does not retain
the de-duplicating property present in the original implementation.
This implementation does a little more work, but retains that property.
* Explicitly choose sp_std Vec and BTreeSet
Ensures that the macro still works if someone uses it in a context
in which sp_std is not imported or is renamed.
* explicitly use sp_std vectors throughout compact macro
This removes the `dev-dependency` onto `sp-npos-elections` from itself.
A crate should not depend on itself directly, especially not to make any
macros work.
Peter Goodspeed-Niklaus