The data that we will serialize and deserialize.
The dep-graph is serialized as a sequence of NodeInfo, with the dependencies specified inline. The total number of nodes and edges are stored as the last 16 bytes of the file, so we can find them easily at decoding time.
The serialisation is performed on-demand when each node is emitted. Using this scheme, we do not need to keep the current graph in memory.
The deserialization is performed manually, in order to convert from the stored sequence of NodeInfos to the different arrays in SerializedDepGraph. Since the node and edge count are stored at the end of the file, all the arrays can be pre-allocated with the right length.