Draft: externalise lwt exit

Context

Lwt-exit has been released. This MR removes it from the source tree and points opam at the released version.

Note that:

• it will speed up the CI ever so slightly because it removes all the tests for the externalised library
• it will decrease the test coverage percentage because it externalises well-tested code

Depends on opam-repository!132 (closed)

Manually testing the MR

If you manage to convince opam to work for you, then everything should be unchanged.

Checklist

• (N/A) Document the interface of any function added or modified (see the coding guidelines)
• (N/A) Provide automatic testing (see the testing guide).
• (N/A) Add item in the Development Version section of CHANGES.md (only for new features and bug fixes).

Reviewers

@pirbo