Printers for paths and services

In the error-handling code for the mockup and proxy client modes, we need to print services. We hacked a function val print_service : type p q i o. (_, _, p, q, i, o, _) Service.t -> string and a val print_path : type pr p. (pr, p) Resto.Internal.path -> string list but it would be probably better to have this functionality done cleanly in resto.