add ctgrind-like testing of CodePoint::decode_secret #600

Job	Run time
Smoke-test fuzzing targets	55s
Clippy	4s
Build + test (stable, ubuntu-latest)	23s
Format	7s
Build + test (beta, ubuntu-latest)	42s
msrv (ubuntu-latest)	2m 49s
Check semver compatibility	9s
Build + test (nightly, ubuntu-latest)	32s
msrv (macos-latest)	2m 38s
Build + test (macos-latest, stable)	11s
msrv (windows-latest)	3m 31s
Check side-channels on base64 decoder	26s
Build wasm32	16s
Build + test (windows-latest, stable)	1m 28s
	14m 11s

Provide feedback