This adds at least one memory allocation (`collect` into a `Vec`) but is still faster for a reasonable number of IDs.
There is a performance penalty (+13%) but this collection enables cheap deletions.
When creating an index (`IndexedReader::create_index`) the unused `Blob`s are not copied into memory, which really improves performance for this operation (5x faster).
This allows filtering elements and finding their dependencies by decoding as few blobs as possible.