Add to_be_a_bag family

I want to use some `Test::Deep` expectations such as `cmp_bag` on `Test::Kantan`.
In this pull request, I implemented `to_be_a_bag` that equals `cmp_bag`.

I considered 3 approaches to achieve the purpose;

  * Add an expectation like `to_be_a_bag`
  * Implement the Plugin mechanism, and add a Test::Deep plugin
  * Add a "how-to-use with Test::Deep" to README

I choose the first approach, because this approach has 4 advantages over otherwise.

  * Easier to be understood (User should not know what is `Test::Deep`)
  * Users do not spend much cost if Test::Kantan take the Plugin mechanism
  * Mejor test frameworks (rspec, chai, ...) intend to provide rich expectations
  * Low-cost implementation