Fix Pair.init and add .fromRaw constructor

src/pair.zig

@@ -31,11 +31,16 @@ pub fn Pair(comptime ctx: anytype) type {
         world: *World,
         raw: c.ecs_id_t,
 
+        pub fn fromRaw(world: *World, relation_: c.ecs_entity_t, target_: c.ecs_entity_t) Self {
+            return .{ .world = world, .raw = c.ecs_pair(relation_, target_) };
+        }
+
         /// Build a pair from the specified `relation` and `target` entities.
-        /// The specified entities must be alive in the world.
+        /// Returns an error if either of the entities is not alive.
         pub fn init(relation_: Entity, target_: Entity) !Self {
-            const raw = c.ecs_make_pair(relation_.ensureAlive().raw, target_.ensureAlive().raw);
+            try relation_.ensureAlive();
-            return .{ .world = relation_.world, .raw = raw };
+            try target_.ensureAlive();
+            return fromRaw(relation_.world, relation_.raw, target_.raw);
         }
 
         /// Ensures this `Pair` is valid and its `relation` and `target`