Add methods for converting between stdlib and ad-hoc SockAddr

As mio is concerned with few memory allocations, two versions of each function
are provided: consuming (no clone()) and using reference (with clone()).

Suggestions are welcome about naming from_std vs consume_std.
I was in doubts. My intuition said "yes", common rule about not doing premature optimisation said "no" :) I guess here it won't harm, so I'll update it.