Avoid confusion with class methods and protected/private modifiers

Working on something else, I saw the following:

protected

# ...

def self.field_type
  to_s.split("::").last.underscore
end

This code can lead to misunderstanding, as the protected modifier doesn't apply to class methods like self.field_type.

On this PR, I move two such class methods behind the modifier, near the top of the class, to avoid this confusion.

I also move three other class methods to the top of the class, as I feel class methods should go together in general, in order to clarify interfaces and avoid future instances of this anti-pattern.