Add a couple of builtin functions, and the Vm::call() method

* Builtin functions print and println have been added
* If a global lookup fails, the VM will attempt to look up a builtin
* Vm::call(fun, args) allows interrupting the current execution state
  and starting a new function instead. It will return the value left on
  top of the stack.

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

src/vm/frame.rs

@@ -93,6 +93,10 @@ impl NativeFrame {
         &self.fun_ptr
     }
 
+    pub fn args(&self) -> &Vec<ObjRef> {
+        &self.args
+    }
+
     pub fn callee(&self) -> &ObjRef {
         &self.callee
     }