Allowing `ifelse-value` as proper expression (fixes #113)
Fixing labels to be dumped before getting stored into updates

Additionally:

  * Adding tests for this case (involving lists) that would fail prior to this commit
  * Renaming some label tests to have more-descriptive names
  * Adding label tests for links
Updating out-of-date type signatures
Fixing the case where multiple ties exist between the same two turtles
Getting `turtlesOwnNames` and `linksOwnNames` off of `World`

Dumps updates incoming
Updating dumps in wake of previous commit
Updating Headless dep. to include tests for recently-fixed things