Integrate status information at arti-client

This commit combines status update information from tor-dirmgr and
tor-chanmgr in the arti-client crate, so that the user can get to
it; it represents a high-level view of the client's ability to reach
the network and route traffic.

I have omitted the tor-circmgr support for now; it's mostly not
needed.

At present it's not so useful, since there's no way for a client to
get a TorClient that _isn't_ completely bootstrapped, and therefore
there's no way to actually watch these events until they're no
longer interesting.  That should change with arti#293.

This is part of #96.