Rework heed to be lightweight and simpler to maintain

[Kerollmops]

• [x] Create a sub-crate for lmdb-sys in this repository, based on the mdb.master.3 branch of LMDB.
• [x] Move from zerocopy to bytemuck (also related to licensing). https://github.com/meilisearch/heed/issues/82
• [ ] Be able to use MDB_WRITEMAP with heed to create databases. https://github.com/meilisearch/heed/pull/116
• [x] Remove the generic types of the RoTxn and RwTxn. https://github.com/meilisearch/heed/issues/94
• [ ] https://github.com/meilisearch/heed/issues/88. This is related to bytemuck. Don't forget to make heed::Error: Send + Sync.
• [x] Reintroduce a faster Database::len function. https://github.com/meilisearch/heed/issues/56
• [ ] Close all the issues/PRs that are related to MDBX as we no more support it and write about that in the README. https://github.com/meilisearch/heed/pull/125#issuecomment-1111910573
• [ ] Delete all of the branches that are not interesting anymore, and move from a master to a main branch.
• [ ] Document the safety issues with the unnamed database. https://github.com/meilisearch/heed/issues/40
• [ ] We should rework the usage of the OwnedType combined with BEU32 types too!