Rework heed to be lightweight and simpler to maintain

[Kerollmops]

• Create a sub-crate for lmdb-sys in this repository, based on the mdb.master.3 branch of LMDB.
• Move from zerocopy to bytemuck (also related to licensing). Moving from zerocopy to bytemuck #82
• Be able to use MDB_WRITEMAP with heed to create databases. MDB_WRITEMAP fix #116
• Remove the generic types of the RoTxn and RwTxn. Remove the generic types of the RoTxn and RwTxn #94
• Support custom encoding/decoding errors #88. This is related to bytemuck. Don't forget to make heed::Error: Send + Sync.
• Reintroduce a faster Database::len function. Improve the poly/database length function #56
• Close all the issues/PRs that are related to MDBX as we no more support it and write about that in the README. Remove mdbx support #125 (comment)
• 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. Document the safety issues with the unnamed database #40
• We should rework the usage of the OwnedType combined with BEU32 types too!