PolyFunction
extends ObjectMarker trait for polymorphic function types.
This is the only trait that can be refined with a polymorphic method,
as long as that method is called apply, e.g.:
PolyFunction { def apply[T_1, ..., T_M](x_1: P_1, ..., x_N: P_N): R }
This type will be erased to FunctionN.