Update model solution.

- Stop calculating operands for & operators.
- Evaluate the lhs of compound operators only once.
- Add `side-effect.c` example.

examples/c/side_effect.c

@@ -0,0 +1,13 @@
+int g = 0;
+
+int* foo() {
+    g += 10;
+    return &g;
+}
+
+int main() {
+    // `foo()` should be called once.
+    *&*foo() += 1;
+
+    return g;
+}