Initial version of Tuple implementation.

__I still need to add proper exception handling.__ The implementation is also inefficient (currently treating integer positions as strings)

This involved more code than I expected.

(e.g. still need to add checks for ranges of positional arguments in Tuple)

Code coverage is incomplete, not fully sure of what methods TupleValue should expose

@lucaswoj what was UnspecifiedTupleType needed for? Are these the right things to start implementing?