views.Slice are meant to be immutable, and if used as such it
is at times desirable to use them as a key in a map. For non-viewed
slices it was kinda doable by creating a custom key struct but views.Slice
didn't allow for the same so add a method to create that struct here.
Updates tailscale/corp#17122
Signed-off-by: Maisem Ali <maisem@tailscale.com>