perf(pet-fs): memoize norm_case to avoid repeated GetLongPathNameW sy… #73

Job	Run time
Performance Baseline (ubuntu-latest, x86_64-unknown-linux-musl)	3m 25s
Performance Baseline (windows-latest, x86_64-pc-windows-msvc)	7m 12s
Performance Baseline (macos-latest, x86_64-apple-darwin)	5m 29s
	16m 6s

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