GuardUsage: restrictions: Use list builder

Although these do not appear in the config, it seems sensible to get rid of what I think is the remaining ad-hoc list manipulation site.

define_list_config_builder requires that the builder element type be Deserialize. Currently GuardUsageRestriction is a transparent, public enum, so we aren't really exposing anything.

We could introduce GuardUsageRestrictionBuilder now, but since it's not in the config and thereofore only in the public API of the lower crates, we can definitely put that off.

This is part of somewhat related to #447 (closed) and therefore #371 (closed) and #285 (closed)

Found with

git-ls-files | xargs perl -ne 'next unless m/^\#.*derive.*Builder/..m/^\}/; print "$ARGV:$.:$_"' 2>&1 |less +/':.*Vec'

crates/tor-circmgr/src/path/exitpath.rs

@@ -160,7 +160,8 @@ impl<'a> ExitPathBuilder<'a> {
                     family.insert(*exit_relay.id());
                     // TODO(nickm): See "limitations" note on `known_family_members`.
                     family.extend(netdir.known_family_members(exit_relay).map(|r| *r.id()));
-                    b.push_restriction(tor_guardmgr::GuardRestriction::AvoidAllIds(family));
+                    b.restrictions()
+                        .push(tor_guardmgr::GuardRestriction::AvoidAllIds(family));
                 }
                 let guard_usage = b.build().expect("Failed while building guard usage!");
                 let (guard, mut mon, usable) = guardmgr.select_guard(guard_usage, Some(netdir))?;