Bring Yaks::Resource::Link to 100% mutation coverage
Refactor Yaks::Mapper::Link
Be more consistent, use symbols to look up attributes. Bring link back to 100% mutcov
First stage of some major refactorings. Objectives

* get to 100% mutcov
* get rid of profile_registry and rel_registry
* make more use of the Policy object
* make dependencies more explicit instead of passing options all over
  the place
Get rid of ProfileRegistry, RelRegistry, MapLinks, SharedOptions, make the tests pass again
Improve tests for associations, make collaborators more explicit, i.e. no more options, just rel, mapper, policy