Refactor ParThreadsafe to use secret-superclass

[rrnewton]

"Secret superclass" is a better design pattern than this current business of using an unsafeParIO method, which we make sure the user cannot instance themselves.

Should be real easy to solve this because unsafeParIO uses can just be replaced with internalLiftIO. And it removes the confusing redundancy between those two.

[osa1]

Hm, I'm confused about how this is supposed to work. If we define 'ParThreadSafeSecretin a non-exported module inpar-classeswe can't implementParThreadSafeat all, becausepar-classesdoesn't implement any instances. If we exportParThreadSafeSecretinpar-classes`, then users can implement instances.

[rrnewton]

The distinction is between Control.Par.Class (SafeHaskell Trustworthy), and Control.Par.Class.Unsafe (SafeHaskell Unsafe).

The user can get to everything with the unsafe module, but they lose SafeHaskell Safe (inferred) status.