Add Ipv6Addr::is_ipv4_mapped This change consists of cherry-picking the content from the original PR[1], which got closed due to inactivity, and applying the following changes: * Resolving merge conflicts (obviously) * Linked to to_ipv4_mapped instead of to_ipv4 in the documentation (seems more appropriate) * Added the must_use and rustc_const_unstable attributes the original didn't have I think it's a reasonably useful method to have. [1] https://github.com/rust-lang/rust/pull/86490