code: public final <K> Ix<Map<K, T>> collectToMap(IxFunction<? super T, ? extends K> keySelector) {
          IxFunction<T, T> f = IdentityHelper.instance();
          return this.collectToMap(keySelector, f);
      }
docs: Collects the elements of this sequence into a Map where the key is
  determined from each element via the keySelector function; duplicates are
  overwritten.
  <p>
  The result's iterator() doesn't support remove().
  @param <K> the key type
  @param keySelector the function that receives the current element and returns
  a key for it to be used as the Map key.
  @return the new Ix instance
  @throws NullPointerException if keySelector is null
  @since 1.0