[Issue 1846] bitmanip.BitArray.opIndex need const read-only version

[Janice Caron]

> There's going to be many const annotations to
> do throughout the library...

Including
    const int opCmp(in Object o)
    const bool object.opEquals(in Object o)
    const hash_t object.toHash()
    const string object.toString()

in Object?

Please...?