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
✓	✨ New feature

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[sbourdeauducq]

This breaks abstraction layers, HasEnvironment does not otherwise know about kernels.
Note that NAC3 does not use kernel_invariants.

[charlesbaynham]

This breaks abstraction layers, HasEnvironment does not otherwise know about kernels. Note that NAC3 does not use kernel_invariants.

True. Is there a better place for it you can think of?