Since the `visit_*` methods are specific to linters, it seems inappropriate to define them in a utility class.

Also, this adds a layer of indirection which I'm not entirely sure I'm comfortable introducing as a pattern to `scss-lint`. Having all necessary code in a linter, while perhaps not [DRY](https://en.wikipedia.org/wiki/Don't_repeat_yourself), makes it easy for you to understand what's going on without needing to refer to externally defined modules. (i.e. when I look at `HexLength` for the first time and I _don't_ see any `visit_*` methods defined, I'm confused as to how it actually works--It's not obvious that I should look at `HexUtils`).

If there is an opportunity for deduplication, I would rather see a helper method defined in the `Utils` module, which is already available to all linters.