Added ability to check the size of a reference map

[ianunruh]

Also added aliases for those of us that prefer method access over array notation

[jdantonio]

@ianunruh Do you still use this gem? The original author recently moved ref to the ruby-concurrency organization so that we (the ruby-concurrency team) can take over maintenance. The ref gem is now a dependency of concurrent-ruby so we plan to begin actively developing and maintaining ref. If you still use ref and are interested in helping maintain it we'd love to have your help.