Add fuzzy matcher.

This is step towards rspec/rspec-expectations#280.  I decided to implement this in rspec-support because this is a general fuzzy-matching algorithm that doesn't depend on any rspec-expectations APIs and because I think we can use it to replace [rspec-mocks' ArgumentListMatcher](https://github.com/rspec/rspec-mocks/blob/master/lib/rspec/mocks/argument_list_matcher.rb).