GuardUsage: restrictions: Use list builder

Although these do not appear in the config, it does have a builder.
It seems sensible to get rid of this ad-hoc list manipulation site,
and replace it with our standard list builder API.

define_list_builder_helper 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.