Add two new type constructors: MethodReturnType and StaticMethodReturnType

[magnus-madsen]

After Native add:

  /**
    * A type constructor that represents the _return type_ of a Java method.
    *
    * A method return type can be resolved when the receiver object and argument types are known.
    *
    * A few examples:
    *
    * - The type: `Apply(InvokeMethod("length", 0), String)` is equivalent to `Int32`.
    * - The type: `Apply(Apply(InvokeMethod("startsWith", 1), String), String)` is equivalent to `Bool`.
    * - The type: `Apply(Apply(Apply(InvokeMethod("substring", 2), String), Int32), Int32)` is equivalent to `String`.
    *
    * The type constructor requires at least one type argument: the type of the receiver object.
    */
  case class MethodReturnType(name: String, arity: Int) extends TypeConstructor {
    def kind: Kind = Kind.mkArrow(arity + 1)
  } 

  /**
    * A type constructor that represents the _return type_ of a Java static method.
    *
    * A static method return type can be resolved when the argument types are known.
    *
    * A few examples:
    *
    * - The type: `Apply(InvokeStaticMethod(String.class, "valueOf", 1), Bool)` is equivalent to `String`.
    * - The type: `Apply(InvokeStaticMethod(String.class, "valueOf", 1), Char)` is equivalent to `String`.
    *
    * The type constructor does not require any arguments.
    */
  case class StaticMethodReturnType(clazz: Class[_], name: String, arity: Int) extends TypeConstructor { 
    def kind: Kind = Kind.mkArrow(arity)
  }