Compact DEI variant.

[cmeiklejohn]

Right now, the DEis are very verbose, which is useful for debugging and other types of analysis, but given that the header length restriction in gRPC metadata is 8192 bytes, it probably means that it will only support graphs that are depth < 6 or so. Therefore, we need the ability to generate a variant of DEI's that are compact: presumably the same algorithm, just a final digest placed on the value. This could be a variant that just extends V1, called V1, which adds an additional digest in the serialization step.