QV4::PropertyKey is a bit of a mess

PropertyKey is a limited version of QV4::Value. We should privately inherit from Value and only expose the limited set of methods we need. Also, there is a hole at UINT_MAX, which is a bit of a mess. In many cases, we use it in this way:

    uint index = foo.asArrayIndex();
    if (index < UINT_MAX) .....

It'd be just as readable to do a isArrayIndex first, and then get the index (which might even do an assert on isArrayIndex).