Introduce default-setting prime for collective (#5137)

* Introduce default-setting prime for collective

* Docs.

* Elections phragmen supports prime

* Fix

* Membership supports prime

* Fix

* Update frame/collective/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/frame/support/src/storage/generator/value.rs

@@ -91,6 +91,14 @@ impl<T: FullCodec, G: StorageValue<T>> storage::StorageValue<T> for G {
 		unhashed::put(&Self::storage_value_final_key(), &val)
 	}
 
+	fn set(maybe_val: Self::Query) {
+		if let Some(val) = G::from_query_to_optional_value(maybe_val) {
+			unhashed::put(&Self::storage_value_final_key(), &val)
+		} else {
+			unhashed::kill(&Self::storage_value_final_key())
+		}
+	}
+
 	fn kill() {
 		unhashed::kill(&Self::storage_value_final_key())
 	}

substrate/frame/support/src/storage/mod.rs

@@ -73,6 +73,10 @@ pub trait StorageValue<T: FullCodec> {
 	/// Store a value under this key into the provided storage instance.
 	fn put<Arg: EncodeLike<T>>(val: Arg);
 
+	/// Store a value under this key into the provided storage instance; this uses the query
+	/// type rather than the underlying value.
+	fn set(val: Self::Query);
+
 	/// Mutate the value
 	fn mutate<R, F: FnOnce(&mut Self::Query) -> R>(f: F) -> R;