Group-Based Optimization of Potent and Cell-Active Inhibitors of the Von Hippel-Lindau (Vhl) E3 Ubiquitin Ligase: Structure-Activity Relationships Leading to the Chemical Probe (2S,4R)-1-((S)-2-(1-Cyanocyclopropanecarboxamido)-3,3-Dimethylbutanoyl)-4-Hydroxy-N-(4-(4-Methylthiazol-5-Yl)Benzyl)Pyrrolidine-2-Carboxamide (Vh298)