- Rewrite route metadata lookup so TS can understand it better, and to
   explicitly handle the case where an expected route is not found in
   the `/api/` response. We don't expect this to happen in normal
   usage but it would provide a more obvious error if eg. something
   intercepted the `/api/` request or a modified fork of h removes a
   route that the client needs.

 - Rewrite `stripInternalProperties` helper to use `Object.entries`
   rather than a `for..in` loop

 - Set the default type to `void` for request and response bodies (ie.
   assume no request/response body unless specified).

 - Adjust types and casts for APICall. To typecheck with `noImplicitAny`
   we need to cast the generic `APICall<...>` to the type for a specific
   route.