Add stack printing debug builtin

This function will print the stack without modifying it.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/builtins.rs

@@ -144,6 +144,15 @@ impl MachineBuilder {
             }
         });
 
+        self.register_builtin_fun("S", |machine, _reentry| {
+            println!("- TOP");
+            for (i, obj) in machine.stack().iter().enumerate() {
+                println!("{}. {:?}", i, obj);
+            }
+            println!("- BOTTOM");
+            Ok(BuiltinExit::Return)
+        });
+
         /*
         //
         // ==