Check the use on type operators

[nibro]

There is a problem in handling type operators. We can't check that TypeOperators? are enabled directly when we encounter them, since we don't know whether the type we're parsing will be used as a data constructor, where the operators would be allowed even in Haskell98. So we need to allow the operators always, and then check in a post-pass that the types are actually valid.

This will probably be implemented in the same check that does forall-checking, for ticket #29.

This is part of ticket #14.

[nibro]

With the new separation between parse types and actual types, this was trivial to implement.