Improve complexity of CompactAssignments::unique_targets (#8314)

* 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