Comprehensive Link support

This implements #267 with a complete, well-typed API for constructing sane Link headers.
Do you mean that this file is cluttered or the package `unfiltered.response.link`? If you mean the file, I agree and it makes sense to do that as the couple of parameter type hierarchies don't gain anything from being `sealed` (the are extensible per the spec and I don't actually use the sealed type anywhere).

If you mean the package then I'm not sure I understand. What is the import tax issue?