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

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

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust