Decapsulate currently allocates a lot and this appears to have been causing some
issues (hard to tell, pprof is misbehaving).
Note: I removed the TODO as this function now *only* checks if we're dealing
with a loopback address. Not sure what `OverIPLoopback` was supposed to mean.
Note 2: Decapsulate actually checked if the IP6 loopback addresses
appeared *anywhere* in the multiadder. However, as we weren't doing this for
IP4, I decided to simplify this and only check prefixes.
Currently IsIPLoopback returns true for any multiaddr starting ip4/127, but only returns true on ip6 addresses that exactly equal ip6/::1. So /ip6/::1/tcp/4001 for example does not return true. Instead we should check whether ip6/::1 is contained in the multiaddr, and to do that I simply decapsulated and checked to see if the result is different. It seems this was not caught by tests as none are present specifically for this function.
