Derive `eq` and `show` for all exposed types

[aronerben]

Deriving eq and show makes it possible for users to easily derive eq and show for their own types that depend on Sihl types. Sihl already depends on ppx_deriving, but certain sihl-* services don't, so this would incur additional dependencies for those.

[timohuber]

Deriving user with sexp (https://github.com/janestreet/ppx_sexp_conv) would be useful, too. We currently use it to store the current user in Opiums context, what requires a sexp_of_t function : https://rgrinberg.github.io/opium/rock/Rock/Context/Key/index.html