Import set 1.0.0

- SortedSet has been removed for dependency and performance reasons.
- Set#join is added as a shorthand for `.to_a.join`.
- Set#<=> is added.

https://github.com/ruby/set/blob/v1.0.0/CHANGELOG.md

lib/set/set.gemspec

@@ -0,0 +1,25 @@
+Gem::Specification.new do |spec|
+  spec.name          = "set"
+  spec.version       = "1.0.0"
+  spec.authors       = ["Akinori MUSHA"]
+  spec.email         = ["knu@idaemons.org"]
+
+  spec.summary       = %q{Provides a class to deal with collections of unordered, unique values}
+  spec.description   = %q{Provides a class to deal with collections of unordered, unique values}
+  spec.homepage      = "https://github.com/ruby/set"
+  spec.license       = "BSD-2-Clause"
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+
+  spec.metadata["homepage_uri"] = spec.homepage
+  spec.metadata["source_code_uri"] = spec.homepage
+  spec.metadata["changelog_uri"] = "https://github.com/ruby/set/blob/v#{spec.version}/CHANGELOG.md"
+
+  # Specify which files should be added to the gem when it is released.
+  # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
+  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
+    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
+  end
+  spec.bindir        = "exe"
+  spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
+  spec.require_paths = ["lib"]
+end

lib/set/sorted_set.rb

@@ -0,0 +1,6 @@
+begin
+  require 'sorted_set'
+rescue ::LoadError
+  raise "The `SortedSet` class has been extracted from the `set` library." \
+        "You must use the `sorted_set` gem or other alternatives."
+end