Nucleophilic Substitution in the Allyl System of Codeine and Pseudocodeine in Their Reactions with Lithium Methyl‐ and Arylcyanocuprates. Crystal and Molecular Structure of 8α‐Phenyl‐ 6,7‐didehydro‐4,5α‐epoxy‐3‐methoxy‐17‐methylmorphinan and 6β ‐Phenyl‐7,8‐didehydro‐4,5α‐epoxy‐3‐methoxy‐17‐methylmor