Fix Facet Indexing bugs

[loiclec]

1. Handle keys with variable length correctly

Closes (partially) https://github.com/meilisearch/meilisearch/issues/3042 This issue is now easily reproducible with the updated fuzz tests, which now generate keys with variable lengths.

2. Prevent adding facets to the database if their encoded value does not satisfy valid_lmdb_key.

Closes (partially) https://github.com/meilisearch/meilisearch/issues/2743 This fixes an indexing failure when a document had a filterable attribute containing a value whose length is higher than ~500 bytes. For now, this fix is just meant to prevent crashes. Better handling of long values of filterable attributes will be handled in a separate PR.

[bors[bot]]

Build succeeded:

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• Tests on macos-latest
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• Tests on windows-latest