fs: accept all valid utf8 values in fast paths #4350

Job	Run time
Build slim tarball	2m 2s
aarch64-linux: with shared libraries	18m 38s
aarch64-darwin: with shared libraries	20m 11s
x86_64-linux: with shared libraries	22m 41s
x86_64-darwin: with shared libraries	28m 28s
	1h 32m 0s

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