Nodetypes design and implementation

[teodosin]

Complete the future-proof design and implementation of node types. The requirements are roughly so:

• A thorough, complete NodeType struct that serialises smoothly into Agdb's DbValue
• Ability to hierarchically categorise node types together (eg. operators, types of operators, files)
• Account for the differences and overlap between NodeTypes and DataTypes (the type of data that a node outputs, sometimes different per output socket)
• Ergonomic way of defining new node types, both internally in the crate and for hypothetical plugin developers
• Support for defining input and output sockets, their data types, and validating connections to those sockets
• NodeType "origins" to differentiate nodes defined by Karta and by different plugins
• Each unique node type should be indexed so that instances of each type can be easily found throughout the whole db. See #96 for similar issue