Lwt 5.0.0, Lwt_ppx 2.0.0

Breaking

See #584 for an extended summary and discussion of this release as a whole, or individual issues for each change specifically.

• The callback passed to Lwt.async must now evaluate to unit Lwt.t, rather than _ Lwt.t (#603, requested @cfcs).
• Lwt.choose, Lwt.nchoose, Lwt.nchoose_split, Lwt.pick, and Lwt.npick now raise Invalid_argument if given the empty list (#557, Tim Reinke).
• Catch nested calls to Lwt_main.run (#607, #609, prompted François-René Rideau).
• Use the new Lwt_unix.IO_vectors in Lwt_unix.recv_msg and Lwt_unix.send_msg (#594, prompted Marcello Seri).
• Make Lwt_unix.Async_switch a synonym for Lwt_unix.Async_detach (#572).
• Remove the redundant configure.ml (#700).
• PPX: remove support for general [%lwt ...] expressions (#527).
• PPX: remove support for Lwt_log and the -log option (#520).
• PPX: remove the -no-debug option (#528).

Bugs fixed

• libev detection under esy (#755, Antonio Nuno Monteiro).