features: populate cop_features from a hints hash the hard way

features: populate cop_features from a hints hash the hard way

I originally optimised populating cop_features for eval by storing
the hints mask in "feature/bits" and then fetching that when
re-populating the hints for eval.

But that has turned out to be too fragile, so iterate over the
possible feature keys and populate cop_features from that.

I could perhaps have avoided this cost by ensuring "feature/bits" was
set where else it was needed, but this code already iterates to build
the hints hash, iterating again doesn't increase the scale of the work
we're doing.