Use reference counting for identifiers

src/Free/Arg.c

@@ -4,7 +4,6 @@ void _GroundFreeArg(GroundArg* in) {
     if (in->type == GroundArg_Value) {
         Ground.Free.Value(&in->as.value);
     } else {
-        // FIXME - double free for arg
-        // free(in->as.ref);
+        Ground.Free.Identifier(in->as.ref);
     }
 }

src/Free/Identifier.c

@@ -0,0 +1,9 @@
+#include "../../include/ground.h"
+
+void _GroundFreeIdentifier(GroundIdentifier* identifier) {
+    identifier->referenceCount--;
+    if (identifier->referenceCount == 0) {
+        free(identifier->string);
+        free(identifier);
+    }
+}