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

substrate/primitives/npos-elections/compact/src/assignment.rs

@@ -125,7 +125,7 @@ pub(crate) fn into_impl(count: usize, per_thing: syn::Type) -> TokenStream2 {
 						let target = target_at(*t_idx).or_invalid_index()?;
 						Ok((target, *p))
 					})
-					.collect::<Result<Vec<(A, #per_thing)>, _npos::Error>>()?;
+					.collect::<Result<_npos::sp_std::prelude::Vec<(A, #per_thing)>, _npos::Error>>()?;
 
 				if sum >= #per_thing::one() {
 					return Err(_npos::Error::CompactStakeOverflow);