introduce rivbin binary encoder (taken from tfchain encoding contribution)

[GlenDC]

Currently we have https://github.com/threefoldtech/rivine/tree/master/encoding, as the binary encoder for the Rivine project. It was taken from the original Sia fork and has remained active ever since.

tfchain (a Rivine-powered blockchain) introduced a new binary encoder, which you can currently find at https://github.com/threefoldfoundation/tfchain/tree/697475d32acde1c00fe7c94d02113ce2f205afbf/pkg/encoding. This will be the new Rivine binary encoder, which we can place under /pkg/encoding/rivbin. What is now under /encoding will be moved to /pkg/encoding/siabin. siabin will still be the default and only used binary encoder by Rivine. However, all primitive types will already support the rivbin encoder as well, such that other chains can already make use of it, should they want.

In a future issue we would than still need to decide how we could enable chains to use rivbin as the binary encoder for all purposes.