API calls now represented by proper types

In this release, one of the main criticisms was addressed: the API calls are now represented by their own types, so there are now channel types, user types and many more. Some parameters can have only a limited number of values, these are also represented using their own types so calling the methods with incorrect values is impossible.

The code was updated to use the new Lwt 2.4.6 ppx macros instead of CamlP4, so this is the minimum required release. Also, better use OCaml 4.02.0 for improved support for ppx.