Syscall: Add support for deduplicating callbacks via function ptr #51

Job	Run time
Test with Go 1.18.10 on FreeBSD	3m 28s
Test with Go 1.18.x on ubuntu-latest	1m 31s
Test with Go 1.19.12 on FreeBSD	4m 7s
Test with Go 1.19.x on ubuntu-latest	1m 9s
Test with Go 1.20.7 on FreeBSD	4m 10s
Test with Go 1.20.x on ubuntu-latest	1m 34s
Test with Go 1.21.0 on FreeBSD	4m 10s
Test with Go 1.21.x on ubuntu-latest	1m 27s
Test with Go 1.18.x on macos-latest	1m 43s
Test with Go 1.19.x on macos-latest	1m 14s
Test with Go 1.20.x on macos-latest	2m 12s
Test with Go 1.21.x on macos-latest	2m 10s
Test with Go 1.18.x on windows-latest	1m 31s
Test with Go 1.19.x on windows-latest	3m 6s
Test with Go 1.20.x on windows-latest	2m 47s
Test with Go 1.21.x on windows-latest	2m 51s
	39m 10s

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