[split] Mcrypt deprecated

Good Day. 

I really appreciate all the effort and the updates were making in the CI 3. Thank you.

Recently, I updated the XAMPP and switch PHP version from 7.0 - 7.1. the Problem is the mcrypt is already depreciated. and by 7.2 it will be removed completely. Moreover, it resulted to an error in the Library/Encryption.php . I  think CI community need to patch it up for the future PHP version that we will be using. the first solution I had is to revert my xammp back to the original and using the PHP 7.0 or update the Encryption.php. I was thinking to use different library for the Encryption. or someone who could update the Encryption.php. .

The error are the following:
Message: Function mcrypt_list_algorithms() is deprecated -236
Message: Function mcrypt_module_open() is deprecated - 269
Message: Function mcrypt_enc_get_algorithms_name() is deprecated - 262
Message: Function mcrypt_module_close() is deprecated - 266
Message: Function mcrypt_module_open() is deprecated - 269

This problem does not and has not ever existed - CI_Encryption has always defaulted to openssl instead of mcrypt.

There's 2 ways to end up using mcrypt instead of openssl and both would be of your own making:

- Manually specify the driver as 'mcrypt'
- Somehow not have the openssl extension, which is enabled by default and so virtually every PHP installation should have it