New API

[clo4]

Some requirements for the new API

• Needs to track how many times a route is fetched
  • An assert to ensure it's fetched exactly, at least, or at most, N times
• Needs ability to un-mock a route after fetching it N times

It might be easier to vendor the dependency on router for this so there's no duplication! Can also use a different syntax, instead of GET@/... it can use GET /...