permit the use of arrays as tuples

[davidchambers]

Commit message:

It's not currently safe to use arrays as tuples due to a bug in the circular-reference detection in _determineActualTypes. Given a value such as ['abc', Right(123)] in an environment containing both Array and Pair (the type comprising all two-element arrays), the function is applied recursively to the nested values once for Array and once for Pair. This is fine in the case of 'abc'; for Right(123), though, this is problematic. The first time, Right(123) is appended to $seen. The second time, the identical object is found in $seen, erroneously indicating a circular reference.

This commit removes the mutation.