Also add a new engine - ENGINE_EVAL. Previously ENGINE_INTERNAL
had three different modes - a "hi-optimized" version, a
"lo-optimized" version and a version that didn't depend on
create_function and there wasn't a way to really isolate these
modes and test them individually.
- rename \phpseclib\Crypt\Base to \phpseclib\Crypt\Common\SymmetricKey
- create BlockCipher and StreamCipher to extend SymmetricKey
- replace _string_shift with \phpseclib\Common\Functions\Strings::shift