add IP.Unspecified?

[Brad Fitzpatrick]

I was just porting some code from net.IP to netaddr.IP and discovered we don't have netaddr.IP.IsUnspecified like std.

Add it?

/cc @danderson @josharian @mdlayher

[Matt Layher]

+1 from me, would save me a couple of ip == net.IPv6Unspecified() anyway.

[Dave Anderson]

Sure. Make sure the docs make it very clear how IsUnspecified and IsZero differ.