Lwt is now MIT-licensed

By popular request, the license of Lwt is now MIT :slight_smile:

Lwt was previously licensed under a combination of LGPL, a linking exception, and BSD, which could be confusing to deal with in cases where licensing really matters. We hope the MIT license makes Lwt a bit easier on users.

The MIT license applies to future releases of Lwt, and to master starting today.

Thanks to all the contributors who have given permission, and participated in the discussion on the switch!