This change makes the allocation of DataTokens and Values on RouteData
lazy, and elides copies when copying an 'empty' RouteData.
In our current architecture this change will eliminiate 2 * (N + 1)
dictionary allocations/copies per request, where N is the number of routes
processed. In a large system with lots of attribute routes, this number
could be very significant.
For a small MVC site (ModelBinding, Validation, Views) with one route, it
still shows a modest reduction of dictionary allocations without adding
much complexity.
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