add Ipv6Addr::to_ipv4_mapped
add Ipv6Addr::to_ipv4_mapped

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