case-folding.rb: conversion script

* enc/unicode/case-folding.rb: script to convert CaseFolding.txt,
  tranlated from CaseFolding.py.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46266 b2dd03c8-39d4-4d8f-98ff-823fe69b080e