std: Second pass stabilization for boxed
          #20420
        
          
      
                
     Merged
            
            
          
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This commit performs a second pass over the
std::boxedmodule, taking thefollowing actions:
boxedis now stableBoxis now stableBoxAnyis removed in favor of a directimpl Box<Any>Box::downcastremains unstable while the name of thedowncastfamily ofmethods is determined.
This is a breaking change due to the removal of the
BoxAnytrait (note thatthe
downcastmethod still exists), and existing consumers ofBoxAnysimplyneed to remove the import in their modules.
[breaking-change]