A couple of function things..

• Functions (and aggregations) should be typed on their families rather than specific PTypes.
• FnSignature (and AggSignature) should be removed in favor of the interface methods.
• Functions (and aggregations) should have an implementation provider rather than a single invoke method.
• Use type families rather than PTypes
• Use a computeReturnType method like Calcite for function typing (massively simplifies PlanType logic...)

    /**
     * TODO REPLACE ME WITH `getInstance(args: Array<PType>)` which returns an invocable instance of this function.
     *
     * Invoke the function with the given arguments. Required.
     *
     * @param args the arguments to the function
     * @return the result of the function
     */
    public fun invoke(args: Array<Datum>): Datum