Allow computing costs via a 5-fn

[NBKelly]

I'm not sure if this would nuke the engine or not, but it would be practical at times to be able to compute the cost from a 5-fn, ie for femme fatale:

:yes-ability {:cost [(->c :credit (-> context :ice :subroutines count))]

Currently, you need to dereference everything for variable/state-based costs, which is ugly and kind of hard to read.