This patch rebases @jgallagher's to the tip of the main branch,
adding support for IPv6 multicast groups. As in the original PR,
it is only possible to join groups by sending an initial MLDv2
Report packet. It is not yet possible to resend the change report,
leave groups, respond to queries, etc.

Also, modify `test_join_ipv6_multicast_group()` so that it joins
the mDNS multicast group instead of 0xfe80::1. Because the lower 24
bits of 0xfe80::01 match the lower part of the IPv6 address assigned
to the test device by `crate::tests::setup()`, `has_multicast_group()`
would always returns `true`.

No part of the code uses this anymore.

- Handle payload copy outside of mld::AddressRecordRepr::emit()
- On unit test, use the parsed Hop-by-Hop option instead of checking
  that the wire representation of the two structures is the same
- Minor code changes

- Split mldv2_router_alert() in two, adding a new function named
  push_padn_option(u8) to emit a PadN hop-by-hop option after any
  other options have been pushed already
- When emiting ReportRecordReprs, remove the conditional block
  that attempted to copy payload data over the destination record