Add signatures for `Set`

[ksss]

I added several signatures for methods in Set. All method tests are testing by raap.

• Aliases
  • #<=
  • #<
  • #>
  • #>=
    • ruby 2.0
    • https://github.com/ruby/ruby/commit/aa7dc0f305c11c2ef9e67f6d48acbd1264022024
  • #filter!
    • ruby 2.6
    • https://bugs.ruby-lang.org/issues/13784
• Methods
  • #compare_by_identity?
    • ruby 2.4
    • https://bugs.ruby-lang.org/issues/12210
  • #join
    • ruby 3.0
    • https://bugs.ruby-lang.org/issues/16991
  • #flatten!
    • From the beginning of Set