Node Storage
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 - `NodeStorage` is the central concept that defines what a node storage must be able to do.
 - `NodeStorageVTable` is a vtable for a NodeStorage. It can be generated from any class that is a `NodeStorage`.
 - `DynNodeStoragePtr` is a non-owning pointer to any `NodeStorage`, it stores an instance-pointer and a vtable-pointer.
 - Identifiers for Nodes and their properties are found in :doc:`t_NodeBackendHandle`
 - A reference implementation of a thread-safe node storage backend based on `tsl::sparse_map` is provided
   at :class:`rdf4cpp::storage::reference_node_storage::ReferenceNodeStorageBackend`
 - :doc:`t_node_storage_view` contains proxy classes to access information about a node stored in an implementation-specific
   backend.


Requirements for `NodeStorage` Implementations
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 - Implementations must not assume that all nodes are stored in the node storage,
   in fact small `Literal`\s that can fit in 42 bits (e.g. values of `xsd:int`) are usually *not* stored there
 - `IRI`\s, `BlankNode`\s and `Variable`\s should usually be stored
   as their string representations
 - `Literal`\s can be stored either as their canonical lexical form or as values
    - when trying to store `Literal`\s as values the implementation is
      required to accurately report which `Literal` types can be stored as
      values via :func:`rdf4cpp::storage::NodeStorage::has_specialized_storage_for`. **Inaccurate
      reporting will result in undefined behaviour.**
    - Warning: `Literals` that have value storage are assumed to never need escaping when converting them to their
      n-triples string representation. I.e. the output of their corresponding `to_string` function will be used
      without escaping it.
    - see :class:`rdf4cpp::storage::reference_node_storage::SyncReferenceNodeStorage` for details
 - Some `IRI`\s are reserved by default (see :var:`rdf4cpp::datatypes::registry::reserved_datatype_ids`)
   these `IRI`\s *must always* be present in the `NodeStorage`
   and assigned the given `NodeID`. **Not upholding this invariant results in undefined behaviour.
   See :class:`rdf4cpp::storage::reference_node_storage::SyncReferenceNodeStorage`
   for the expected usage.**

General Notes for Implementors
______________________________

 - **Passing a `Literal` that is supposed to be stored in lexical storage (as required by the `NodeStorage` implementation)
   as a value view results in undefined behaviour.**
 - **Passing a `Literal` that is supposed to be stored in value storage (as required by the `NodeStorage` implementation)
   as a lexical view results in undefined behaviour.**