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
since the `TagHelperExpression<T>` case will be far less common than simple types and `TagHelperExpression` derivatives, need to test `int`, `string`, some random complex type, and direct `TagHelperExpression` derivatives.