Rollup merge of #87708 - the8472:canonical_v6, r=dtolnay

Add convenience method for handling ipv4-mapped addresses by canonicalizing them

This simplifies checking common properties in an address-family-agnostic
way since #86335 commits to not checking IPv4 semantics
of IPv4-mapped addresses in the `Ipv6Addr` property methods.

diff --cc library/std/src/net/ip.rs
Simple merge