Solution: remove implementation. Frank Hartmann <soundart@gmx.net>,
the author, rejected our request to relicense under MPL2, so we
have to remove his copyrighted work.
Tweetnacl is not security-supported and could not be used in
production environments anyway, the supported backend is libsodium.
Solution: switch to Github Actions
Travis recently started enforcing credits for OSS projects without
any funding. While it is possible to get free credits, it is a manual
step that involves contacting customer support via email and asking to
add them, every week. While this does not require money, it requires
something far scarcer: volunteers time.
Drop Travis and migrate to Github Actions.
