Simplify some filesystem extensions on Windows #297

Job	Run time
Rustfmt	10s
Check Cross-Compilation on Rust nightly (nightly)	1m 49s
Check Cross-Compilation (stable)	2m 9s
Test on Musl (ubuntu)	27s
Check Cross-Compilation (beta)	2m 12s
Test (stable)	11s
Test with Rust nightly (ubuntu)	35s
Test (windows)	41s
Test (macos)	25s
Test with Rust nightly (windows)	1m 10s
Test (macos-11.0)	25s
Test (beta)	19s
Test (ubuntu-20.04)	13s
Test (windows-2019)	1m 38s
Test (aarch64-ubuntu)	1m 47s
	14m 11s

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