config derive attrs: Make builders serde, and validated structs not

 * Builders additionally derive: Debug, Serialize, Deserialize.

 * Validated structs no longer derive: Serialize, Deserialize
   and all related attributes deleted.

 * As a consequence, all the `#[serde(deny_unknown_fields)]`
   are gone.  That means that right now unknown fields are totally
   ignored.  This is good for compatibility but poor for useability.
   Doing something better here is arti#417, in progress.

 * As a consequence, delete tor_dirmgr::retry::default_parallelism.
   (The default value was already duplicated into a builder attr.)