unix/birthtime: use broad support of libstd (#100) #136

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Job	Run time
Rustfmt	8s
Publish Documentation	32s
Build for Android and Redox (x86_64-linux-android)	18s
Test (stable)	10s
Build for Android and Redox (x86_64-unknown-redox)	8s
Test (beta)	15s
Test (nightly)	12s
Test (macos)	31s
Test (win32)	51s
Test (win64)	1m 43s
Test (mingw)	32s
	5m 20s

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