Prepare release 0.4.0

[mmagician]

Description

Prepare steps:

• [x] merge latest master to releases
• [x] branch out to release/0.4.0
• [x] bump versions (& dependencies versions)

Once this PR is merged to releases:

• [ ] (potentially not needed in curves) remove dev-dependencies from all crates (due to circular dev-deps). Ideally done in a single commit.
• [x] release the crates and tag the versions (run cargo update; git tag v0.4.0; git push origin v0.4.0; cargo release publish --execute --verbose)
• [ ] (potentially not needed in curves) add back the dev-deps (revert the commit done above)
• [ ] finally merge everything to master, so that crates in master are 0.4.0