`Refreshable.map` operates on the read-side so is less efficient than it could be

[bastewart]

As in the title, the implementation of Refreshable.map means that every read of the cache involves a transformation of the data structure. We should probably consider implementing this instead through caching the value post-transformation (or adding new method to do this). This could/would cost more memory, but is probably worth it!

Not urgent at all though, I can just do the map on my side pre-caching.