Switching to big-endian byte order in `pkg/io/serialize.go`?

[sebastianst]

While writing a proposal for perun-proposals, I realized that, in general, we use little-endian byte order in pkg/io (see variable byteOrder in file serialize.go). But for big.Ints, we use Byte() which returns the big int in big-endian byte order. So in order to be more consistent, we should consider changing the byteOrder to BigEndian. After reading a bit about byte orders, it also seems that big endian is still more commonly used in networking/wire protocols (whereas little endian byte order is the winner in chip architectures).

The only implication of this change would be that existing persisted channel data would be invalid.