Graph and map serializer

[Alpar Juttner]

For real distributed computing, it would be nice to have a serializer for the graphs and the map structures.

By a serializer I mean a facility to represent a graph/map as a compact raw sequence of bytes that can be sent to another (local or remote) process and the graph/map can be reconstructed from it.

The raw data format should be compact and easy to code and decode, however it cannot be independent from the underlying graph representation, as we must keep the node/arc IDs at the graph reconstruction.

This proposal is somewhat related to #225.