CI improvements
CI improvements

- Run test suite on more targets
  NetBSD and WASI are now explicitly tested for compatibility. Illumos
  will be tested once it lands on stable.
- Merge no_std and std tests
  This reduces the amount of configuration necessary at little extra
  cost.
- Use cargo-hack to test all feature combinations
  This does significantly increase the time needed for a successful CI
  run. However, it ensures that any combination is functional. To avoid
  extremely long builds, only `--lib` tests are run for the powerset.
  `--doc` tests are run only on the full set.
- Reintroduce caching
  The corrupted cache is due to the behavior of GitHub Actions, and has
  an issue filed for it. In the meantime, using GNU tar apparently
  avoids corruption. As such, it is installed on macOS when appropriate.

time-rs/time