Update enum_like_derive to syn 0.15

The api has changed so it isn't as simple as bumping the version number.

While we are at it, investigate if it's worth it to support complex expressions like

```rust
enum Number {
    Zero = (0, 1).0,
    One = (0, 1).1,
}
```

(see also: https://github.com/illicitonion/num_enum)