Methylated Analogues of Methyl ( R )-4-(3,4-Dichlorophenylacetyl)- 3-(pyrrolidin-1-ylmethyl)piperazine-1-carboxylate (GR-89,696) as Highly Potent κ-Receptor Agonists: Stereoselective Synthesis, Opioid-Receptor Affinity, Receptor Selectivity, and Functional Studies