Release 3.1.8

• Don't use native types for invalid literals.
• Disallow @reverse within an embedded node (flattening can't handle it).
• Update flatten algorithm to handle embedded nodes and annotation objects.
  • Perform bnode renaming before creating the node map to handle embedded nodes. Adds test option to do result comparison by remapping bnode labels based on dataset bijection.
• Make renaming bnodes default to false for Reader and to_rdf_spec.
• Fix expansion bug where a property is @reverse and the same subject has an explicit @reverse.
• Don't compact an @id or @value to a string if it contains an annotation.
• Make sure list members don't have annotations.