Improve mocks of builtin macros

A post to the arduino developers' mailing list suggests a smarter way to do macros for (e.g.) `max` using "statement expressions":

```c++
#define max(a,b) \
  ({ typeof (a) _a = (a); \
      typeof (b) _b = (b); \
    _a > _b ? _a : _b; })
```

Adopting this will prevent double-evaluating `a` and `b` if for some reason they themselves are expressions with side effects.