Add world.pair function

src/world.zig

@@ -17,6 +17,7 @@ pub fn World(comptime ctx: anytype) type {
         const Context = @import("./context.zig").Context(ctx);
         const Entity = Context.Entity;
         const Iter = Context.Iter;
+        const Pair = Context.Pair;
 
         raw: *c.ecs_world_t,
 
@@ -118,6 +119,18 @@ pub fn World(comptime ctx: anytype) type {
             return lookupAlive(self, Context.lookup(T));
         }
 
+        /// Looks up and returns a `Pair` made from the specified relation
+        /// and target entities, returning an error if either is not alive.
+        ///
+        /// Note that in most cases, for functions that accept an `anytype`,
+        /// you can simply specify the tuple `.{ relation, target }` to
+        /// specify a pair, without having to use this function.
+        pub fn pair(self: *Self, relation: anytype, target: anytype) !Pair {
+            const rel = try self.lookupAlive(Context.anyToEntity(relation));
+            const tar = try self.lookupAlive(Context.anyToEntity(target));
+            return Pair.fromRaw(self, rel.raw, tar.raw);
+        }
+
         /// Creates or modifies an `Entity` in this `World`.
         /// See `Entity.init(...)` for more information.
         pub fn entity(self: *Self, config: Entity.Config, add: anytype) !Entity {