Validate cargo package contents in CI

[joshlf]

Add a CI test that runs cargo package or cargo publish --dry-run or similar, and validates certain aspects of the resulting artifacts. This should include:

• Validate that the LICENSE file was included, and that its contents are correct
  • This is especially important since zerocopy-derive's license file was spuriously removed in [derive] Remove stale LICENSE symlink #275 and re-added later in [derive] Include LICENSE file when publishing #489

This issue requires both identifying what contents we should be checking for and then writing the test. It's okay if this work is done incrementally (e.g., first checking for a single file, then adding more checks over time).