Remove/Refactor Set and Util classes

[tom-acciarresi]

The Set class is only ever used once in the code base and can be easily removed or merged with Util

Having this plugin occupying such generic class names as "Util" and "Set" pollutes the global namespace

These classes should be removed/trimmed down/implemented as inside classes of other classes (e.g. Util.transpose_vec should become a private function inside TerrainPreset)

[pablogila]

Seems reasonable, have you tested it?

[tom-acciarresi]

Seems reasonable, have you tested it?

I've made the changes on my personal repository and it works just fine.
Just haven't found an opportunity to clean it up for an actual PR.