Add Set#intersect? and #disjoint?.

* lib/set.rb (Set#intersect?, Set#disjoint?): Add new methods for
  testing if two sets have any element in common.
  [ruby-core:45641] [Feature #6588] Based on the code by marcandre.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42253 b2dd03c8-39d4-4d8f-98ff-823fe69b080e