Support scalaCheck for properties

[vkuncak]

ScalaCheck can provide better bug finding in some cases compared to the current state of Stainless.
Furthermore, there are likely various users of it.
Hence, it would be very helpful to define Stainless library with API compatible with ScalaCheck, but supporting proofs as well.

Particularly interesting might be support for a hybrid between instantiating some values and keeping the others symbolic. For example, we could instantiate data structure shape, but leave the content symbolic.