net/netip: use slice-to-array conversions

net/netip: use slice-to-array conversions

Use slice-to-array conversions in AddrFromSlice and
(*Addr).UnmarshalBinary. This allows allows to use AddrFrom16 and drop
the redundant ipv6Slice helper.

For #46505

Change-Id: I0e3a7d8825ad438115b7f23ee97cc74eec41a826
Reviewed-on: https://go-review.googlesource.com/c/go/+/432735
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>