Add ICMPv6 Echo Request/Reply Handling to `SNat6`

[FelixMcFelix]

Currently, ICMPv4 echo flows interact with source NAT by allocating themselves a port in the SNAT range, then mapping this port to/from the correct echoID. This has not been extended to ICMPv6 – this code was originally written as a body transform, and ICMPv4 was explicitly left unparsed to enable this. Now that ICMPv4 is treated as a full-fledged ULP (#408), it should be straightforward to extend the newer header transform to ICMPv6.

This should also present some opportunity to cleanup similarities and identical code between SNat and SNat6.