Taught export to produce, and import to understand, a table-of-contents (toc) file.
Emitted 'next to' the export file or files, named -toc.json.
Consists of keys "meta" and "files". "files" is a dictionary of export-file/checksums. "meta" contains the "file", "chunk_size", and "global_hash" of the export.
Added toc= to import. Import will find and validate the checksums of any chunk_files, reassemble them into a single .tar.gz, and pass that along to the rest of the import process. Deletes chunks as it goes, to minimize disk-space.
Updated import-export docs to describe TOC file and its use.