Set instance variables of the singleton class

[ashmaroli]

• Initialize singleton class instance_variables instead of using private attr_writer methods — since those private methods don't seem to be used elsewhere consequently.
• Define attr-* of the singleton class at the top of the class block (similar to how attr-*s for regular class instances are defined)
• Correct comment to reflect the fact that :lax is the default error_mode instead of :warn.
• Insert whitespace into comments to improve their readability.

/cc @dylanahsmith, @tjoyal, @davidcornu