Last updates and fixes before dropping support for ocaml 4.02.3
- For tuple contains more than 2 elements, fix the element order when deriving rpcty
- pythongen, rpc_genfake: improved failures for Abstract types
- cmdlinergen: implement Abstract
- add support for ocaml 4.06
- Remove unnecessary warnings
- Add failable tests for ocaml-4.06
- opam: add upper bound to async
- opam: remove [configure] from the build stanza