Add support for Floret embeddings

• Add support for reading, writing, and using Floret embeddings.
• Add a finalfusion chunk type for Floret-like vocabularies.
• Add support for batched embedding lookups (embedding_batch and embedding_batch_into)
• Improve error handling:
  • Mark wrapped errors using #[source] to get better chains of error messages.
  • Split Error::Io into Error::Read and Error::Write.
  • Rename some Error variants.