Decide on card and atom API

[elucid]

Cards and atoms are currently both implemented as modules with #name, #type, and #render defined as module_functions.

This isn't great Ruby style. Here are some alternatives:

• using classes instead of modules would require class-level method definitions of #name and #type so that searching and validation work in a reasonable way. this is kind of messy but would be somewhat nicer if card and atom base classes were provided (possibly using something like cattr_accessor for #name...)
• using a hash with render being a callable. this seems sloppy.

Also, I'm not in love with the long method signatures for render. Considering switching to

  def render(*args)
    env = args[:env]
    value = args[:value]
    ...
  end