variables defined using def in a let statement now escape local scope

test added as well.

src/sym.rs

@@ -79,6 +79,10 @@ impl SymTable {
         self.0.get(arg)
     }
 
+    pub fn contains_key(&self, arg: &String) -> bool {
+        self.0.contains_key(arg)
+    }
+
     pub fn insert(&mut self, k: String, v: Symbol) -> Option<Symbol> {
         self.0.insert(k, v)
     }