This change optimizes allocations by RouteValueDictionary based on usage.
First, implement a struct Enumerator, and expose the concrete RVD type
from all extensibility points. We wanted to try and decouple this code
from RVD originally and use IDictionary everywhere. After doing that we've
found that it allocates an unacceptable number of enumerators.
Secondly, optimize copies of RVD for the case where you're copying an RVC
to another (common case). When doing this we can copy the count to get the
right capacity, and copy the entries without allocating an enumerator.
Lastly, optimize RVD for the case where it's a wrapper around a poco
object. We 'upgrade' to a writable full dictionary if you try to write to
it, or call one of a number of APIs that are uncommonly used. We could
produce optimized versions of things like `Keys` and `CopyTo` if necessary
in the future.
d4b96b27c0