arti-client, dirmgr: Initial DirFilter code

This is a prerequisite for #397 (closed); I'm giving review to @Diziet since he's working on config changes and we've been discussing the right API for this.

This code adds a DirFilter type (behind a feature gate) for dirmgr, so that directory objects can be modified before they're used by the rest of Arti. I'll use this for testing purposes, by creating munged directory objects where e.g. all the relays have the wrong onion key or no exits exist.