Os_types: use submodule User and Group.

[dannywillems]

PR #264
A module Os_types is used to define only types like userid, groupid, user, etc to be able to use them in all modules to have clearer interfaces and avoid circular dependencies.

For the moment, we have Os_types.userid, Os_types.groupid, Os_types.user, Os_types.group and Os_types.activationlinkkey_info in this module.

As this module will grow in the future (more types will be added), I think it will be better to define submodules and have Os_types.User.id, Os_types.User.t, Os_types.Group.id, Os_types.Group.t and Os_types.Activation_link_key.info. I think this method is clearer because if we want to add more types related to users, we add a type in the submodule User and we won't have something like Os_types.user_new_types_1, Os_types.user_new_types_2, etc.

I come back on OAuth2 and OpenID Connect (see #10) and I defined lots of different types (server_id, client_id, oauth2_client, oauth2_server, etc) used in different modules and it's very ugly if I prefix all things with Os_types.oauth2_client_foo_bar_baz.