Adding support for subscripts to qualified names. This also removes the QN...

Adding support for subscripts to qualified names.  This also removes the QN copy constructor and adds an assert to ensure that the no attribute/no subscript QN constructor does not receive any strings with '.', '[', or ']'.  Additionally this changes the self.qn construction to be a tuple of (base QN, attribute/subscript) instead of a concatenation of the base QN and attribute/subscript so that the has_attr and has_subscript fields are handled properly.  Constant subscripts are not yet supported.

PiperOrigin-RevId: 188111933