The properties on TemplateRoute for DataTokens and Defaults are now
readonly. This prevents modifying these collections in a way that
invalidates cached data, or violates thread-safety.
To do the same for constraints, this change includes a substantial refactor
of how we realize inline constraints, and moves the constraint resolver
out of the parsing phase.
This allow creates a builder for the constraint map, that will make it
easier to implement features like optional constraints, and is reusable
for anyone building their own type of routing system.
Doug Bunting
Ryan Nowak
Ben Brown