agnosticdev/CheaperIPv6Address: Provide cheap(er) IPv6Address init

[agnosticdev]

Provide an interface to create a IPv6Address without taking a heap hit from using an array.
This change exposes the a tuple of UInt32 that can be used to extract a IPv6Address value from the C-struct value sockaddr_in6.
For example:

extension IPv6Address
    static func getAddressTuple(_ sockaddr_in6: sockaddr_in6) -> (UInt32, UInt32, UInt32, UInt32) {
        #if os(Linux)
        return (
            sockaddr_in6.sin6_addr.__in6_u.__u6_addr32.0,
            sockaddr_in6.sin6_addr.__in6_u.__u6_addr32.1,
            sockaddr_in6.sin6_addr.__in6_u.__u6_addr32.2,
            sockaddr_in6.sin6_addr.__in6_u.__u6_addr32.3
        )
        #elseif canImport(Darwin)
        return (
            sa.sin6_addr.__u6_addr.__u6_addr32.0,
            sa.sin6_addr.__u6_addr.__u6_addr32.1,
            sa.sin6_addr.__u6_addr.__u6_addr32.2,
            sa.sin6_addr.__u6_addr.__u6_addr32.3
        )
        #endif
    }
}