HasEnvironment: Ensure that kernel_invariants always exists

[charlesbaynham]

ARTIQ Pull Request

Description of Changes

A simple usability tweak. Ensures that kernel_invariants always exists, so that user code doesn't need to add the usual boilerplate of self.kernel_invariants = getattr(self, "kernel_invariants", set()) before each addition.

Related Issue

Type of Changes

	Type
✓	:sparkles: New feature

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• [x] Use correct spelling and grammar.
• [ ] Update RELEASE_NOTES.rst if there are noteworthy changes, especially if there are changes to existing APIs.
• [ ] Close/update issues.
• [x] Check the copyright situation of your changes and sign off your patches (git commit --signoff, see copyright).

Code Changes

• [ ] Run flake8 to check code style (follow PEP-8 style). flake8 has issues with parsing Migen/gateware code, ignore as necessary.
• [ ] Test your changes or have someone test them. Mention what was tested and how.
• [ ] Add and check docstrings and comments
• [ ] Check, test, and update the unittests in /artiq/test/ or gateware simulations in /artiq/gateware/test

Documentation Changes

• [ ] Check, test, and update the documentation in doc/. Build documentation (cd doc/manual/; make html) to ensure no errors.

Git Logistics

• [ ] Split your contribution into logically separate changes (git rebase --interactive). Merge/squash/fixup commits that just fix or amend previous commits. Remove unintended changes & cleanup. See tutorial.
• [ ] Write short & meaningful commit messages. Review each commit for messages (git show). Format:

  topic: description. < 50 characters total.
  
  Longer description. < 70 characters per line
  

Licensing

See copyright & licensing for more info. ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.