[Port from CoreCLR] Implement slicing APIs for ArraySegment<T>

[jamesqo]

This is a port of ArraySegment's slicing APIs from dotnet/coreclr#9926.

/cc @jkotas

[jamesqo]

@jkotas BTW, I see you guys have a shared folder in coreclr, presumably to share sources between there and corert. Should ArraySegment<T> go there?

[jkotas]

Thanks!

I see you guys have a shared folder in coreclr, presumably to share sources between there and corert. Should ArraySegment go there?

Yes. We have been reconciling diffs between CoreCLR and CoreRT, and moving the files over to the shared folder. The shared folder is automatically mirrored between CoreCLR and CoreRT. We are 40-50% done.