Find type classes with type params but without evidence args

[yannbolliger]

Until now, type class instances with type parameters but without evidence arguments would not be created. This PR refactors the evidence argument search and fixes that problem.

As an example, take this impl:

impl<T> Default for Option<T> {
  fn default() -> Option<T> {
    Option::None
  }
}

This will be extracted as a type class case class OptionTasDefault[T]() extends Default[Option[T]] and it should always be possible to just create an instance of it.

Additionally:

• Improve the error message returned from invoking stainless.
• More test cases for type class extraction were added.