This pr implement the intermediary document representation for milli. The JSON, JSONL and CSV formats are replaced with the format instead, to push the serialization duty on the client side.

The documents module contains the interface to the new document format:

• The DocumentsBuilder allows the creation of a writer backed document addition, when documents are added either one by one, or as arrays of depth 1. This is made possible by the fact that the seriliazer used by the add_documents methods only accepts [Object] and Object. The related serialization logic is located in the serde.rs file.
• The DocumentsReader allows to to iterate over the documents created by a DocumentsBuilder. A call to next_document_with_index returns the next obkv reader in the document addition, along with a reference to the index used to map the field ids in the obkv reader to the field names

All references to json, jsonl or csv in the tests have been replaced with the documents! macro, works exaclty like the serde_json::json macro, as a convenient way to create a DocumentsReader.

Rewrote the search cli, to the cli crate, to also allow index manipulation. This only offers basic functionalities for now, but is meant to be easier to extend than http ui

blocked by #308