[WIP] Add set type

This PR is meant to implement the set type and therefor solve #65 .
We acutally _have_ methods to perform some of the tasks, so we don't have to set them to `NULL`:
 * `copyf` can be set to `ws_object_getref()`
 * `freef` can be set to `ws_object_unref()`
 * `hashf` can be set to `ws_object_hash()`