EncodeToInterface and DecodeToInterface methods should be public

[DominikDeak]

The VCDiffEncoder::EncodeToInterface method and the corresponding VCDiffDecoder::DecodeToInterface are currently private in their respective classes.

I don't see the rationale behind this, because the streaming versions of the encoder/decoder classes publicly expose methods that work with OutputStringInterface derived objects.

The aforementioned methods should be made public, because working with OutputStringInterface offers more flexibility.