Right-size the VecDeque in coerce_unsized.
          #50963
        
          
      
                
     Merged
            
            
          
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The default capacity of a VecDeque is 8, which is excessive here. In a
"base incremental" check build of rustc-perf's tuple-stress benchmark,
this decreases total heap allocation by 26%. I couldn't see a clear
speedup, but it can't hurt.