fn: add Filter to List

This commit adds an immutable Filter method to the linked List API.
This is useful because there are several instances wherein we iterate
through the linked List and only process a subset of it in some way
or another.

fn/list.go

@@ -300,3 +300,18 @@ func (l *List[A]) PushFrontList(other *List[A]) {
 		n = n.Prev()
 	}
 }
+
+// Filter gives a slice of all of the node values that satisfy the given
+// predicate.
+func (l *List[A]) Filter(f Pred[A]) []A {
+	var acc []A
+
+	for cursor := l.Front(); cursor != nil; cursor = cursor.Next() {
+		a := cursor.Value
+		if f(a) {
+			acc = append(acc, a)
+		}
+	}
+
+	return acc
+}