code: public static <K, V> EntryStream<K, V> of(K key, V value) {
          return of(Stream.of(new SimpleImmutableEntry<>(key, value)));
      }
docs: Returns a sequential {@code EntryStream} containing a single key-value
  pair
  
  @param <K> the type of key
  @param <V> the type of value
  @param key the key of the single element
  @param value the value of the single element
  @return a singleton sequential stream