Clean up implementation of builtin functions.

builtin.h

@@ -6,7 +6,7 @@
 block builtins_bind(block);
 
 
-typedef void (*cfunction_ptr)(jv input[], jv output[]);
+typedef void (*cfunction_ptr)(void);
 
 struct cfunction {
   cfunction_ptr fptr;
@@ -15,4 +15,7 @@ struct cfunction {
 };
 
 
+jv cfunction_invoke(struct cfunction* function, jv input[]);
+
+
 #endif