PSR-12 compliancy

Accessing a key in an array is not allowed when the array is not a variable but a value returned by a function.

This also required adding an additional argument to
SimpleSAML\Utils\Crypto::loadPrivateKey to ease in testing. Without
this additional argument, SimpleSAML_Configuration::getBaseDir eventually
gets called to determine the private key location.  This doesn't work
well with vfsstream. This argument shouldn't cause too much trouble, and
seems cohesive enough with the function's purpose.

This resolves #476.

Reimplement \SimpleSAML\Utils\Crypto::aesDecrypt() and \SimpleSAML\Utils\Crypto::aesEncrypt() to use openssl instead of mcrypt. This removes the direct dependency on mcrypt and closes #228.

Move the aes[En|De]crypt() functionality in \SimpleSAML\Utils\Crypto to internal methods where the secret is passed as a parameter. Then we don't need a working configuration to test the functionality.

This reverts commit aec9571d.