Prepare for release

Set version in opam file

Bump version numbers in opam files

Update version numbers in opam files

lwt.opam: version is now 5.4.2

Improve CHANGELOG

Lwt 5.4.0 and Lwt+ppx 2.0.2 release

- version bump in opam files
- CHANGELOG

Version 5.3.0

Version 5.2.0

Version 5.1.2