Make .traverse from fn work correctly.

Functions are also valid keys thus harder to disambiguate which method to call
based on argument type.

Necessarily removed 'traversed' argument from .traverse. If you need to supply a
pre-traversed list of nodes, use .traverseAll or .traverseFrom directly.
Correct .traverse arguments switching behaviour.

Reintroduces traversed parameter.
Simplify/rename API, add link traversal.

* Removed canLink/canUnlink bs as no longer needed with before etc.
* Renamed traverse to visit
* Added new 'traverse links' method, named traverse.
* Removed traverseLinked as it was stupid.
Upgrade to performant beforefn 2.2.0
Release 2.0.0