Correct the QuickCheck AST generator to verify that the ASTs are valid wrt to parser restrictions

[achudnov]

Specifically, validLHS and sourceElement.

[achudnov]

Also, might be useful to rewrite the fixUp cruft to use the new restricted generation facility in testing-feat-0.4.*. Specifically, see https://hackage.haskell.org/package/testing-feat-0.4.0.2/docs/Test-Feat-Class-Override.html#t:Override and https://hackage.haskell.org/package/testing-feat-0.4.0.2/docs/Test-Feat-Class.html#t:ConstructorDeriv