added env function and test

src/sym.rs

@@ -87,6 +87,10 @@ impl SymTable {
         self.0.remove(arg)
     }
 
+    pub fn iter(&self) -> std::collections::hash_map::Iter<'_, String, Symbol> {
+        self.0.iter()
+    }
+
     pub fn call_symbol(
         &mut self,
         name: &String,