Make Num coercer demand definite invocant

This changes the error message reported back to user from a cryptic
'Cannot look up attributes in a <...> type object' to more correct
'Invocant of method 'Num' must be an object instance of type 'Rational',
not a type object of type 'Rat'.  Did you forget a '.new'?'

src/core.c/Rational.pm6

@@ -65,7 +65,7 @@ my role Rational[::NuT = Int, ::DeT = ::("NuT")] does Real {
 
     method nude() { $!numerator, $!denominator }
 
-    method Num(--> Num:D) {
+    method Num(Rational:D: --> Num:D) {
         nqp::p6box_n(nqp::div_In($!numerator,$!denominator))
     }