Upgrade to `mio` 0.8 #68

Job	Run time
Windows (unstable, x86_64-pc-windows-msvc)	1m 23s
Windows (unstable, i686-pc-windows-msvc)	1m 27s
Windows (unstable force-inprocess, x86_64-pc-windows-msvc)	1m 30s
Windows (unstable force-inprocess, i686-pc-windows-msvc)	1m 21s
Windows (unstable windows-shared-memory-equality, x86_64-pc-windows-msvc)	1m 37s
Windows (unstable windows-shared-memory-equality, i686-pc-windows-msvc)	1m 30s
Linux (unstable)	55s
Linux (unstable force-inprocess)	42s
Linux (unstable memfd)	53s
macOS (unstable)	1m 11s
macOS (unstable force-inprocess)	1m 23s
Result	1s
	13m 53s

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