Drop requests.core dep from respond, use plain int status
Removes the transitive dependency on ocaml-crypto (C stubs fail
with -fPIC in Docker amd64 builds). Status is now plain int with
reason_phrase lookup. 78 tests still pass.
Will re-add typed Status.t/Method.t when either:
- ocaml-crypto C stubs are fixed for Docker builds, or
- http-types is extracted as a lightweight package