Make FixedSingle default constructor private

Author: mbouaziz

skiplang/prelude/src/skstore/FixedDir.sk

@@ -389,11 +389,13 @@ base class IFixedDir<+T: frozen, RowRepresentation: frozen> protected {
 }
 
 /*****************************************************************************/
-/* Immutable set (represented with an Array), where the elements are
+/* Immutable map (represented with an Array), where the elements are
  * guaranteed to be unique. */
 /*****************************************************************************/
 
-class FixedSingle<K: Orderable, +V: frozen>(data: Array<(K, V)> = Array[]) {
+class FixedSingle<K: Orderable, +V: frozen> private (
+  data: Array<(K, V)> = Array[],
+) {
   static fun empty(): this {
     FixedSingle()
   }