Return a Duration object for subtraction of two DateTime objects

If you subtract two date times return a Duration of the seconds instead of just an Int this should in theory behave exactly the same for everyone involved but just be ready for any additional date functionality added to duration.

lib/DateTime/Math.pm6

@@ -87,7 +87,7 @@ multi infix:«-»(DateTime:D $dt, Numeric:D $x) is export {
 }
 
 multi infix:<->(DateTime:D $a, DateTime:D $b) is export {
-  $a.posix - $b.posix;
+  Duration.new($a.posix - $b.posix);
 }