Simpler `is-even` and `belongs-to` functions

From a strictly syntactic standpoint, the `is-even` function could be written like this:

``` scss
@function is-even($int) {
  @return $int % 2 == 0
}
```

Same goes for `belongs-to`:

``` scss
@function belongs-to($tested-item, $list) {
  @return index($list, $tested-item) != false;
}
```

... and this makes `contains-display-value` easier as well.

Any reason not to do so?