* [Bug #21449] Fix Set#divide{|a,b|} using Union-find structure Implements Union-find structure with path compression. Since divide{|a,b|} calls the given block n**2 times in the worst case, there is no need to implement union-by-rank or union-by-size optimization. * Avoid internal arrays from being modified from block passed to Set#divide Internal arrays can be modified from yielded block through ObjectSpace. Freeze readonly array, use ALLOCV_N instead of mutable array.