说明：  写一个对文本文件加密的程序和一个解密的程序。密码规则是：对于小写字母，a换成x，b换成y，c换成z，d换成a，e换成b，...；对于大写字母，A换成X，B换成Y，C换成Z，D换成A，E换成B，...；其他字符不变。
(Write a text file encryption program and a decryption process. Password rules are: For the lowercase letters, a replaced by x, b replaced by y, c replaced by z, d replaced by a, e replaced by b ,... for capital letters, A replaced by X, B replaced by Y , C replaced by Z, D replaced by A, E replaced by B ,... other characters unchanged.)