Add scene system and update example

engine/scene.scm

@@ -0,0 +1,42 @@
+(module (engine scene) ()
+(import scheme
+	(chicken base)
+	(chicken module)
+	(engine core)
+	(srfi 1)
+	(srfi 99))
+
+;; This is just for easier serialization
+(define-record-type <entity>
+  (make-entity name components)
+  entity?
+  (name entity-name)
+  (components entity-components))
+
+(export entity named-entity)
+(define (entity . components)
+  (apply make-entity (cons '() (list components))))
+
+(define (named-entity name . components)
+  (apply make-entity (cons name (list components))))
+
+(export scene)
+;; Every element of items here is either an entity record or system
+(define (scene . items)
+  (lambda ()
+    (clear-world)
+    (clear-systems)
+    (clear-event-bus 'input)
+    (for-each
+     (lambda (entity)
+       (if (null? (entity-name entity))
+	   (apply create-entity (entity-components entity))
+	   (apply create-named-entity (cons (entity-name entity)
+					    (entity-components entity)))))
+     (filter entity? items))
+    (for-each
+     (lambda (system)
+       (add-system system))
+     (filter system? items))))
+
+)