Consolidate and rename Graphene data types, and add many comments

[Keavon]

• Combine NodeImplementation into DocumentNodeImplementation to use only the latter everywhere that formerly used either (NodeImplementation::DocumentNode became DocumentNodeImplementation::Network)
• Rename DocumentNodeImplementation::Unresolved to DocumentNodeImplementation::ProtoNode
• Split ProtoNodeInput::Node(NodeId, bool) which stored a boolean indicating if it's treated as a lambda, into ProtoNodeInput::Node(NodeId) and ProtoNodeInput::NodeLambda(NodeId)
• Add a good number of comments throughout Graphene