Cache and freeze strings

[kstephensic]

• #__enum_name__ : cache and freeze String.
• #=== : simplify logic.
• Use @ivar ||= ... pattern.
• #name_sym : caching is ~ 15% faster.
• #to_s is implied in String interpolation.
• Add frozen_string_literal: true.
• '#[]' : simplify logic.

Other Ideas, but I didn't want to touch too much:

#name_sym - set it upon model create/load, rather than caching.
#__enum_name__ - dup.freeze model create/load, rather than caching.