[FEATURE] Simplify FormalParam

[konnov]

We have the following three definitions to express operator parameters:

/**
 * A formal parameter of an operator.
 */
sealed abstract class FormalParam extends Serializable {
  def name: String

  def arity: Int

}

/** An ordinary formal parameter, e.g., x used in A(x) == ... */
case class SimpleFormalParam(name: String) extends FormalParam with Serializable {
  override def arity: Int = 0
}

/** A function signature, e.g., f(_, _) used in A(f(_, _), x, y) */
case class OperFormalParam(name: String, arity: Int) extends FormalParam with Serializable {}

It is not clear to me why the parameters are so complex. We can have one class instead:

class OperParam(name: String, arity: Int) extends Serializable {
  def isHigherOrder: Bool = arity > 0

  /* override equals and hashCode */
}

This refactoring will touch a lot of code. But we should do it after having integrated the type checker.