borrow check, wip4

examples/borrow2.rs

 fn f(a: &i32, b: &mut i32) {}
 fn main() {
     let mut a = 1;
-    f(&a, &mut a)
+    f(&mut a, &mut a)
 }

examples/borrow3.rs

+fn f(a: &i32, b: &mut i32) {}
+fn main() {
+    let mut a = 1;
+    let b = &mut a;
+    let c = &mut a;
+    let d = *b + *c;
+}

tests/test_check.rs

@@ -180,3 +180,8 @@ fn borrow() {
 fn borrow2() {
     check(&read_file::parse("examples/borrow2.rs")).unwrap();
 }
+
+#[test]
+fn borrow3() {
+    check(&read_file::parse("examples/borrow3.rs")).unwrap();
+}