Marker trait for polymorphic function types.

This trait can only be refined with a polymorphic method, as long as that method is calledapply, e.g.: PolyFunction { def apply[T_1, ..., T_M](x_1: P_1, ..., x_N: P_N): R } Exactly one term argument list is expected. The term argument list may be contextual.

This type will be erased to FunctionN or FunctionXXL.