hint about calling the function when unifying with a value

This [SO question](https://stackoverflow.com/questions/48875323/int-should-be-void-int-when-comparing-two-integers) got me thinking that we could add something like `Did you forget to call the function?` to the unification error like this:

```haxe
function getId() return 10;
if (getId == 10) { // Int should be Void -> Int
}
```

This seems like a low-hanging fruit.