An http.Handler that routes requests to Rpcs or other Handlers. Stacks are one of two fundamental building blocks of Cav server applications, the other being Rpcs. If a matching handler or Rpc throws the special NO_MATCH error, the stack continues looking for matching handlers to process the request into a response. Stacks can capture path groups while routing the request; the captured groups will become the basis for the ResolverArg's groups property inside a matching Rpc (before parsing).