enable POSTBUILD to take part in object construction

it will be called after everything else has been done. it'll
get the same arguments as the BUILD submethod, but things have
already been initialized, default values populated, ...

src/Perl6/Metamodel/BUILDPLAN.nqp

@@ -98,6 +98,12 @@ role Perl6::Metamodel::BUILDPLAN {
             @plan[+@plan] := [13, $obj, $_.key];
         }
 
+        # Does it have a POSTBUILD?
+        my $postbuild := $obj.HOW.find_method($obj, 'POSTBUILD', :no_fallback(1));
+        if !nqp::isnull($postbuild) && $postbuild {
+            @plan[+@plan] := [0, $postbuild];
+        }
+
         # Install plan for this class.
         @!BUILDPLAN := @plan;