Store references, not implementations, for document nodes

Make `DocumentNode` instances link to their `DocumentNodeDefinition`s instead of storing many redundant copies of the definition

Start by locating DocumentNode and DocumentNodeDefinition, then inspect how document nodes currently store and construct their definitions. Check the relevant tests or add coverage for the storage behavior; done means instances link to definitions without redundant copies and the existing tests pass.

索引モデルが issue の本文から書いたものです。