Combined version of LMDB mdb.master and mdb.master3

[Kerollmops]

This PR implements #51, taking a large inspiration from the im-rs crate .

It uses a second heed3/Cargo.toml with the necessary dependencies. When you need to work or publish the heed3 crate, you need to cp heed3/Cargo.toml heed/Cargo.toml. The examples were moved out of the standard heed/examples/ folder to make them compile when working on both crates.

There are three crates now...

• heed: the one you know. Based on LMDB 0.9 (the mdb.master branch).
• heed3: Based on LMDB 1.0 (the mdb.master3 branch), without support for encryption, will eventually support checksumming.
• heed3-encryption: The same as heed3 but with support for encryption.

...but I want to only have two

• heed: the one you know. Based on LMDB 0.9 (the mdb.master branch).
• heed3: Based on LMDB 1.0 (the mdb.master3 branch), with support for encryption through the EncryptedDatabase and EnvOpenOptions::open_encrypted type and method and eventually support checksumming.

Note that if we duplicate the code of the Database for the EncryptedDatabase type, we no longer need the following proc-macro as we will copy/paste and change the method signature, i.e., &RoTxn -> &mut RoTxn, by hand.

How it works?

By annotating all the heed methods that use a &RoTxn this way:

#[heed_master3_proc_macro::mut_read_txn(rtxn)]
fn get<'t>(&self, rtxn: &'t RoTxn, key: &[u8]) -> heed::Result<&'t [u8]>;

It transforms the function signature to use a &mut RoTxn:

fn get<'t>(&self, rtxn: &'t mut RoTxn, key: &[u8]) -> heed::Result<&'t [u8]>;

This way, we ensure that users do not keep pointers for potentially invalid bytes from LMDB between two get/put operations. It's a limitation of LMDB when you use the encryption feature. The LMDB pages are decrypted on the fly in a buffer that cycles. As a result, only a restricted amount of values' pointers are valid until the next operations.

To Do

• Modify the CI to test the heed3 crate.
• Create and use the lmdb-master3-sys based on the mdb.master3 branch.
• Introduce two EnvOpenOptions/EnvEntry?
• Retrieve the work from Expose the LMDB Encrypt/Decrypt feature #134 for encryption support (and update the dependencies).
• Add an explanation about the fact that there are three crates and why
  • In the README
  • In the documentation
• Can we make the doc tests to pass, or be conditional on the branch?
• Publish the lmdb-master3-sys and heed3 crates on crates.io.
• Add the support for checksumming too (optional)