Speed up Set#flatten

[kyanagi]

Improved performance by ensuring that identical Set objects are processed only once.

benchmark

prelude: |
  set = Set[1, 2]
  mul = Set[set, Set[set, 4], 3]
  large_set = Set.new(1..10000)
benchmark:
  - set.flatten
  - mul.flatten
  - large_set.flatten

compare-ruby: ruby 3.4.0dev (2024-11-29T02:48:10Z master 43dd9c721f) +PRISM [arm64-darwin22]
built-ruby: ruby 3.4.0dev (2024-11-29T02:48:10Z master 43dd9c721f) +PRISM [arm64-darwin22]

Iteration per second (i/s)

	compare-ruby	built-ruby
set.flatten	1.357M	1.767M
	-	1.30x
mul.flatten	470.122k	686.182k
	-	1.46x
large_set.flatten	899.207	905.055
	-	1.01x