To be able to reason about backwards compatibility, downgrade attacks,
and future configurable enforcement of minimum required (security)
features, introduce the concept of a "feature level" in Signed
References.
In this implementation feature levels are assumed to be monotonic, i.e.,
that higher feature levels behave like lower ones, and only add
semantics on top.
Implementing this as an `enum` that is `Ord` flows nicely.
Treatment of the write-side is left for the next commit. Currently,
the write side "promises" `FeatureLevel::Parent`, but is not able
to "keep the promise", since it does not actually inspect the
feature level of the head in case it is unchanged.
CHANGELOG
372a78754a