tor-hsservice: Code motion (abolish svc, ipt_mgr, imports - prelude)

I hope this is a convenient time to have done this. This branch consists of four phases:

1. Abolish tor_hsservice::svc as per #1146 (closed). I ended up making a new module for IptLocalId.
2. Make two changes where we swap one type for a roughly-equivalent one, so that we're using the same one throughout the module. In one case this may be a bugfix.
3. Introduce crate-local prelude. This should be seen as an experiment to see if we like this. Prompted by discussions in #1060, but there's no warning suppression right now. diffstat is -172loc.
4. Move code in ipt_mgr.rs as requested by #1212 (closed)

It's possible that the layout I chose for the prelude isn't the best one. I suggest we iterate on it.