Reimplement DNS identifier validation. #239

Job	Run time
Minimal supported Rust version	6s
unix (ubuntu, stable, master, release)	4m 25s
unix (ubuntu, stable, master, aws-lc-static)	6m 11s
unix (ubuntu, stable, master, aws-lc)	5m 41s
unix (ubuntu, stable, master, debug)	4m 9s
unix (ubuntu, stable, master, boringssl)	6m 42s
unix (ubuntu, stable, stable-1.28, release)	4m 26s
unix (ubuntu, stable, stable-1.22, debug)	4m 6s
unix (ubuntu, 1.81.0, stable-1.28, debug)	5m 38s
unix (ubuntu, stable, master, debug-static)	3m 57s
unix (ubuntu, stable, master, boringssl-static)	7m 6s
unix (ubuntu, stable, stable-1.28, debug-static)	4m 4s
unix (ubuntu, stable, stable-1.28, debug)	5m 9s
unix (ubuntu, stable, stable-1.28, boringssl)	6m 26s
unix (macos, stable, stable-1.28, debug)	7m 7s
unix (ubuntu, stable, stable-1.28, boringssl-static)	7m 28s
	1h 22m 41s

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