Ship 0.4.0

[Shnatsel]

There is a number of fixes that landed since the latest release on crates.io, and users are noticing, see #474

We should ship a new release with these improvements.

• Merge Generate output per target, not per package #441
• Write a changelog
• Bump versions
• Trigger the release workflow on Github Actions

[Shnatsel]

We are going to land the conversion to cargo-metadata first. We have to fix bugs either way, and it's better to put in the effort once, after the conversion, instead of twice.