proto: replace hidden field with From impl #5246

Job	Run time
test (ubuntu-latest, stable)	1m 32s
test on freebsd	3m 54s
test (ubuntu-latest, beta)	1m 23s
test on illumos	6m 29s
test (macos-latest, stable)	1m 55s
test on netbsd	5m 23s
test (windows-latest, stable)	3m 25s
test on solaris	6m 21s
test-aws-lc-rs	1m 54s
audit	21s
lint	1m 7s
msrv	1m 12s
test wasm32-unknown-unknown	48s
features (ubuntu-latest)	1h 25m 18s
test-android (x86_64-linux-android, x86_64, 26)	2m 58s
features (macos-latest)	25m 17s
test-android (x86_64-linux-android, x86_64, 25)	2m 42s
features (windows-latest)	51m 2s
test-android (i686-linux-android, x86, 26)	2m 52s
test-android (i686-linux-android, x86, 25)	2m 29s
	3h 28m 22s

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