Add mutate_expressions method to GenericForStatement (#236)

Author: jiwonz

src/nodes/statements/generic_for.rs

@@ -94,6 +94,11 @@ impl GenericForStatement {
         self.expressions.iter_mut()
     }
 
+    #[inline]
+    pub fn mutate_expressions(&mut self) -> &mut Vec<Expression> {
+        &mut self.expressions
+    }
+
     #[inline]
     pub fn mutate_block(&mut self) -> &mut Block {
         &mut self.block