Cleanup mli files

[mseri]

• idl.mli and rpc_lwt.mli could be further simplified (removing unnecessarily exposed values) and reorganised a bit. To mention some:

  • the types in Rpc_lwt.M could all become abstract as the _ fn type
  • the declare function in the idl modules should return 'a res
  • some exceptions could be exposed to provide a better debugging experience

• rpc_async.mli is missing

When fixing these it would be nice to add some documentation