Make bossbar use `EntityLayer` for visibility #1375

Job	Run time
valence-fmt (ubuntu-latest, default, nightly-2023-06-20)	36s
extractor-tests (windows-latest)	2m 47s
extractor-tests (macos-latest)	3m 8s
extractor-tests (ubuntu-latest)	2m 8s
valence-docs (windows-latest, default, stable)	4m 40s
valence-docs (macos-latest, default, stable)	3m 39s
valence-docs (ubuntu-latest, default, stable)	2m 13s
valence-clippy (windows-latest, default, stable)	4m 25s
valence-clippy (macos-latest, default, stable)	4m 12s
valence-clippy (ubuntu-latest, default, stable)	2m 22s
valence-tests (windows-latest, default, stable)	8m 43s
valence-tests (macos-latest, default, stable)	8m 55s
valence-tests (ubuntu-latest, default, stable)	5m 37s
	53m 25s

Provide feedback