Stable document format MVP

[Keavon]

Stable document format (next 3 weeks MVP):

• Infrastructure for migrating old to new formats
• Separate structure for storing the document, decoupled from the struct DocumentMessageHandler and struct NodeNetwork definitions
  • One struct per version, plus an associated upgrade function that goes to the next version
  • ASCII graph structure markup format (use RON syntax, we need to define the semantics), see Create a user-facing ASCII format for nodes and subgraphs #1605
  • Translation back from the ASCII format to those structs
• A way to look up a built-in node's definition by its name and not have to store its sub-network
• A way to store user-created subgraph network (or modified from a built-in node definition)