Implement ldict-empty? predicate.

doc/utils.md

@@ -49,6 +49,12 @@ populating it with initial data.
 If only one argument is given, the procedure checks whether it is a
 list of equality procedure and acts accordingly.
 
+    (ldict-empty? d)
+
+* ```d``` - a ldict instance
+
+Returns true if given dictionary contains no keys.
+
 ### IO
 
     (import util-io)