Clean up lib/maruku/output/to_markdown.rb and emit a deprecation warning when #to_md is used
Clean up lib/maruku/output/to_s.rb and emit a deprecation warning when #to_s is used
Clean up lib/maruku/output/s5/to_s5.rb
Clean up lib/maruku/output/s5/fancy.rb
Move document.rb out of structures.rb, fold structures_iterators.rb into structures.rb
Clean up extensions