Rollup merge of #119081 - jstasiak:is-ipv4-mapped, r=dtolnay

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] rust-lang/rust#86490

Author: Nadrieril