CI/CD

Upgrade to .NET Standard 2.1 and improve P/Invoke marshaling #27

Job	Run time
build-native (macos-latest, osx-arm64)	21s
build-native (windows-latest, win-x64)	27s
build-native (ubuntu-latest, linux-x64)	23s
pack-and-publish	0s
package-test	0s
publish	0s
	1m 11s

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