Change vm from a closure to a record type for ease of use, expose data and ret stacks

tests.scm

@@ -120,12 +120,12 @@
     (define my-vm (make-vm))
     (vm-load-program! my-vm adder-program-bytecode)
     (vm-pc-set! my-vm 5)
-    ((my-vm 'vm-run))
+    (run-vm my-vm)
     (assert-equal 3 (vm-memory-ref my-vm 1)))
   (define-test "fib"
     (define my-vm (make-vm))
     (vm-load-program! my-vm fib-program-bytecode)
     (vm-memory-set! my-vm 1 10)
     (vm-pc-set! my-vm 5)
-    ((my-vm 'vm-run))
+    (run-vm my-vm)
     (assert-equal 55 (vm-memory-ref my-vm 1))))