utcp

Use `string` instead of `Cstruct.t` for `recv` and `send` (higher-level user API) (#55) -- `handle` still uses `Cstruct.t` * Use strings instead of Cstruct.t for the higher-level recv and send functions (for clients to use for receiving and sending data) * The `handle` function, which is called for received segments, still uses `Cstruct.t` (for various purposes, including checksum computation, and realizing the MirageOS API uses Cstruct.t) --------- Co-authored-by: Hannes Mehnert <hannes@mehnert.org>