Add order-encoded finite-domain variables

[Lipen]

We need to provide more encodings for finite-domain variables. Currently, only OneHot encoding is supported and being used by default. But there many more exist, for example, order (unary) encoding, binary encoding, onehot+binary encoding, mixed radix encoding. At least the most common ones have to be implemented in kotlin-satlib. And we should begin with order-encoding, including the exploration of problems where such encoding is vital/beneficial, and adding the corresponding examples/tests.