Extend frame a bit

src/frame.rs

@@ -4,7 +4,7 @@ use stack::Stack;
 #[derive(Debug)]
 pub struct Frame {
     pub stack: Stack,
-    locals: Vec<i32>,
+    pub locals: Vec<i32>,
 }
 
 impl Frame {
@@ -18,6 +18,14 @@ impl Frame {
         }
     }
 
+    pub fn len(&self) -> usize {
+        self.locals.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.locals.is_empty()
+    }
+
     pub fn get(&self, offset: usize) -> Result<i32> {
         if offset >= self.locals.len() {
             return Err("Local variable out of range");