In “Spanky,” the hero—an unassuming London clerk named Martin—succumbs to temptation and enters into a pact akin to Faust’s. His demonic and completely immoral friend and helper, Spanky, arranges for him an opulent life: money, women, a new apartment, and confidence in himself at last. In return, Spanky seemingly asks only for selfless friendship. But that’s only at first…