As elsewhere, it makes sense when adding or extending a cell type to
actually make the code to parse it into a separate tested function.
This commit doesn't actually make anything use these new functions;
that's for a later commit.
This makes it so we can handle getting an IPv6 in the 3 different
formats we specified it for in RESOLVED cells,
END_STREAM_REASON_EXITPOLICY cells, and CONNECTED cells.
We don't cache IPv6 addresses yet, since proposal 205 isn't
implemented.
There's a refactored function for parsing connected cells; it has unit
tests.
