Tag helpers registration system

- This registration system accepts tag descriptors that it then uses to feed requests for tag helpers.
- Added some infrastructure pieces that are used to build up valid tag helper descriptors.

#70
Yeah. The only scenario that would be affected by removing the Union and using a set instead is when a user registers a helper as both tag specific as well as a catch all. I imagine in that scenario we end up running the helper twice - which might be ok because it's not a very sensible thing to do to start with.