Promote methods to public on relay's array conn
Promote methods to public on relay's array conn

I recently extracted a gem called
[graphql-page_cursors](https://github.com/artsy/graphql-page_cursors)
that extracts Artsy's graphql pagination approach but it relies on all
connections exposing first and last methods. Unless I'm missing
something, I believe in the case of Relay's Array connection these
methods have been marked private by mistake.

rmosolgo/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby