Use frame-decode for decoding storage keys in subxt?

[jsdw]

Perhaps we can simplify Subxt and remove the storage key decoding logic by relying on frame-decode instead?

(side effect: this might also lead to useful frame-decode feedback as well as meaning we only decode things in one place)

If doing this makes everything more complicarted in subxt or proves to not be worth while then no worries, but probably worth a shot!