Seal public Protocol reference types to prevent external inheritance #1853

Job	Run time
build (macos-latest, Release)	4m 24s
build (windows-latest, Debug)	11m 20s
build (macos-latest, Debug)	4m 57s
build (ubuntu-latest, Release)	5m 16s
build (ubuntu-latest, Debug)	5m 27s
build (windows-latest, Release)	12m 35s
publish-coverage / publish-coverage	47s
	44m 46s

Provide feedback