isUnreserved : Char -> Bool
判断字符是否为URL中无需编码的unreserved字符 参考RFC 3986: ALPHA / DIGIT / "-" / "." / "_" / "~"
percentEncode : String -> String
URL编码(百分号编码)实现 将非unreserved字符转为%XX格式
percentDecode : String -> String
URL解码(百分号解码)实现 将%XX格式还原为原字符