Cache author info in Comment classes

[jpslav]

For a spreadsheet with 420 rows and around 150 columns, we were seeing Axlsx serialization times of around 317 seconds. I tracked the problem down to the Axlsx Comment and Comments classes always recalculating author lists and indexes of authors. Our workbook had lots of comments and so these recalculations that might otherwise go unnoticed stood out. In a ruby-prof run, Axlsx::Comments#to_xml_string was taking around 90-95% of the total run time.

Since we have no need to indicate comment authors in our workbooks, I initially monkey patched two methods in Axlsx to fix this:

module Axlsx
  class Comments < SimpleTypedList
    def authors
      [""]
    end
  end

  class Comment
    def author_index
      0
    end
  end
end

This change took our Axlsx time down to 5 seconds.

This PR achieves the same time savings but preserves the existing capabilities by introducing caching into the sorted authors list and into the method that returns the index of an author in that list.

[jpslav]

Removed this line because I'm pretty sure author is already an attribute of this Comment class.