Bump dependencies to latest revisions

[lorentey]

Notably, this updates the swift-system dependency from 0.0.3 to 1.2.1, a version with source stability.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering my changes (if appropriate).
• I've verified that my change compiles and works correctly, and does not break any existing tests.
• I've updated the documentation (if appropriate).

[lorentey]

@swift-ci test