Add fuzzer for `utf8::validate`. #1974

Job	Run time
rustfmt	13s
test (pinned, ubuntu-latest, 1.65.0)	1m 19s
test (stable, ubuntu-latest, stable)	1m 43s
test (beta, ubuntu-latest, beta)	30s
test (nightly, ubuntu-latest, nightly)	24s
test (macos, macos-latest, stable)	48s
test (win-msvc, windows-latest, stable)	1m 5s
test (win-gnu, windows-latest, stable-x86_64-gnu)	1m 21s
	7m 23s

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