Implement new ldict-set procedure.

doc/utils.md

@@ -81,6 +81,15 @@ of missing key.
 Returns a new dictionary with the record under given key removed. If
 the dictionary does not contain the key ```k```, an error is raised.
 
+    (ldict-set ld k v)
+
+* ```ld``` - a ldict instance
+* ```k``` - a compatible key
+* ```v``` - a value to insert
+
+Sets existing key ```k``` to the new value ```v``` or inserts it if it
+is not already present in the dictionary ```ld```.
+
 ### IO
 
     (import util-io)