`dup`ing an Identity Hash/Set is faster than creating a new one

[amomchilov]

It's one less allocation, too :)

	YJIT off	YJIT on
Set.new.compare_by_identity	2.571M	4.260M
S.dup	3.014M	4.999M
∆	1.17x	1.17x

	YJIT off	YJIT on
Hash.new.compare_by_identity	5.643M	8.208M
H.dup	5.988M	9.014M
∆	1.06x	1.09x

S = Set.new.compare_by_identity.freeze
H = Hash.new.compare_by_identity.freeze

Benchmark.ips do |x|
  x.config(time: 10)

  x.report(".new.compare_by_identity") do |times|
    i = 0
    while (i += 1) <= times
      S.dup
    end
  end
  
  x.report(".dup                    ") do |times|
    i = 0
    while (i += 1) <= times
      Set.new.compare_by_identity
    end
  end
  
  x.compare!
end

Benchmark.ips do |x|
  x.config(time: 10)
  
  x.report(".new.compare_by_identity") do |times|
    i = 0
    while (i += 1) <= times
      H.dup
    end
  end
  
  x.report(".dup                    ") do |times|
    i = 0
    while (i += 1) <= times
      Hash.new.compare_by_identity
    end
  end
  
  x.compare!
end

[JuanVqz]

@amomchilov could you please opne a PR?