Build via Swift package manager

[ephemer]

Type of change: DX, Performance

Motivation (current vs expected behavior)

Re-allow builds via SwiftPM, which should make it easier for others to integrate the library into their own builds. We also gain improved support for static linking, whole module optimization, etc. by doing this.

Please check if the PR fulfills these requirements

• [x] Self-review: I am confident this is the simplest and clearest way to achieve the expected behaviour
• [x] There are no dependencies on other PRs or I have linked dependencies through Zenhub
• [ ] The commit messages are clean and understandable
• [ ] Tests for the changes have been added (for bug fixes / features)