Enable compilation with #![no_std] #57

Job	Run time
Test (stable)	2m 17s
Test (beta)	2m 23s
Test (nightly)	2m 40s
Test (linux32)	2m 20s
Test (macos)	22s
Test (aarch64-ios)	26s
Test (win32)	1m 28s
Test (win64)	2m 26s
Test (mingw64)	1m 7s
Test (windows-latest)	2m 20s
Test (stable-no-std, ubuntu-latest, stable, x86_64-unknown-linux-gnu, --no-default-features)	2m 21s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc)	2m 19s
	22m 29s

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