New API

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 /...`