Try parsing signatures only if node is callable, see #149; Minor refactoring

src/definitions.ts

@@ -3,10 +3,13 @@
  * @module definitions
  *//***/
 
+import {
+  CommonFlags
+} from "./common";
+
 import {
   Program,
   Element,
-  CommonFlags,
   ElementKind,
   Global,
   Enum,