Support empty IpcSharedMemory (#335) #101

Job	Run time
Linux	29s
macOS	29s
Windows (x86_64-pc-windows-msvc)	44s
macOS (force-inprocess)	24s
Linux (force-inprocess)	31s
Windows (i686-pc-windows-msvc)	57s
Linux (memfd)	28s
macOS (async)	30s
Windows (--features force-inprocess, x86_64-pc-windows-msvc)	1m 0s
Linux (async)	35s
Windows (--features force-inprocess, i686-pc-windows-msvc)	47s
Windows (--features windows-shared-memory-equality, x86_64-pc-windows-msvc)	49s
Windows (--features windows-shared-memory-equality, i686-pc-windows-msvc)	1m 6s
Windows (--features async, x86_64-pc-windows-msvc)	51s
Windows (--features async, i686-pc-windows-msvc)	1m 3s
Result	1s
	10m 44s

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