Removing redundant compiler case
Fixing bug in setting unbreeded link directedness
Making `LinkManager._linkOfBreed` public, like its `TurtleManager` buddy
Updating to latest version of Headless
Ensuring that links inherit breed variables
Moving the `iterator` method up to `Seq` (with a default)
Improving type annotation
Handling special linksets properly
Fleshing out support for link breeds
Formatting minutiae
Rethinking recent change to `Seq`

Turns out that `AbstractAgentSet` was already doing this!
Doing link comparison better
Making agentset equality ignore item ordering
Implementing `link-set` and `turtle-set`
Making links default to the correct shape
Adding more testing for link breeds