Accpet `OsStr` instead of `str` for flags #1602

Job	Run time
Test CUDA support	38s
Test build-std (aarch64-tvos)	1m 33s
Test build-std (aarch64-tvos-sim)	1m 57s
Clippy	23s
Test build-std (x86_64-tvos)	1m 24s
MSRV (ubuntu-latest)	20s
Rustfmt	9s
MSRV (windows-latest)	38s
semver-checks	17s
Test (stable)	24s
Test (beta)	38s
Test (nightly)	31s
Test (linux32)	1m 30s
Test (macos)	2m 26s
Test (aarch64-macos)	46s
Test (aarch64-ios)	55s
Test (win32)	1m 38s
Test (win64)	3m 1s
Test (mingw32)	1m 34s
Test (mingw64)	1m 29s
Test (windows-2019)	2m 17s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 7s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	2m 14s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	2m 11s
	31m 0s

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