fix VM not clearing memory references when reused (#813)

vm/vm.go

@@ -67,9 +67,11 @@ func (vm *VM) Run(program *Program, env any) (_ any, err error) {
 	if vm.Stack == nil {
 		vm.Stack = make([]any, 0, 2)
 	} else {
+		clearSlice(vm.Stack)
 		vm.Stack = vm.Stack[0:0]
 	}
 	if vm.Scopes != nil {
+		clearSlice(vm.Scopes)
 		vm.Scopes = vm.Scopes[0:0]
 	}
 	if len(vm.Variables) < program.variables {
@@ -614,3 +616,10 @@ func (vm *VM) Step() {
 func (vm *VM) Position() chan int {
 	return vm.curr
 }
+
+func clearSlice[S ~[]E, E any](s S) {
+	var zero E
+	for i := range s {
+		s[i] = zero // clear mem, optimized by the compiler, in Go 1.21 the "clear" builtin can be used
+	}
+}