docs: public IntBiMap union(IntBiMap other) {
      int newLength = keys.length + other.keys.length;
  
      int[] newKeys = Arrays.copyOf(keys, newLength);
      ArrayUtils.copy(other.keys, 0, newKeys, keys.length, other.keys.length);
      int[] newValues = Arrays.copyOf(values, newLength);
      ArrayUtils.copy(other.values, 0, newValues, values.length, other.values.length);
  
      return IntBiMap.fromUnsortedKeyValues(newKeys, newValues);
    }