Refactor into more modular architecture

entity.h

@@ -0,0 +1,20 @@
+#ifndef ENTITY_H
+#define ENTITY_H
+
+#define MAX_ENTITIES 256U
+
+typedef void (*update_cb_t)(unsigned new_entity_id, void *ref);
+typedef void (*remove_cb_t)(void *ref);
+
+void entities_clear();
+void entities_purge();
+
+unsigned entity_add();
+void entity_mark(unsigned id);
+
+unsigned entity_add_component(
+    unsigned id, update_cb_t update, remove_cb_t remove, void *ref);
+void entity_update_component(
+    unsigned entity_id, unsigned component_id, void *new_ref);
+
+#endif