Add all the A tasks

Task/Abstract-type/Racket/abstract-type.rkt

@@ -0,0 +1,13 @@
+#lang racket
+
+(define animal-interface (interface () say))
+
+(define cat% (class* object% (animal-interface) (super-new))) ;; error
+
+(define cat% (class* object% (animal-interface)
+               (super-new)
+               (define/public (say)
+                 (display "meeeeew!"))))
+
+(define tom (new cat%))
+(send tom say)