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 must 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 Two 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), with support for encryption through the EncryptedDatabase and EnvOpenOptions::open_encrypted type and method and will eventually support checksumming.

The heed3 crate uses nearly the same Database signature as the heed crate but it has an EncryptedDatabase that wraps a normal Database and overrides the signature like the following:

// method on the heed Database
fn get<'a, 'txn>(&self, txn: &'txn RoTxn, key: &'a KC::EItem) -> Result<Option<DC::DItem>>;

// the same method but on heed3 EncryptedDatabase
fn get<'a, 'txn>(&self, txn: &'txn mut RoTxn, key: &'a KC::EItem) -> Result<Option<DC::DItem>>;

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 number of entry pointers are valid until the next operation.

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).
• Opening an already opened env returns an error (no longer the env).
• Improve the EnvAlreadyOpened error description.
• Reintroduce the EncryptedEnv/Env::prepare_for_closing method.
• Make the Env clone again.
• Can we make the doc tests to pass, or be conditional on the branch?
• Add an explanation about the fact that there are two crates and why
  • In the README
  • In the documentation
  • Explain how to release
• Publish the lmdb-master3-sys, heed-types and heed3 crates on crates.io Publish heed and heed3 #294.
• Update dependencies Update the heed and heed3 dependencies #293
• Add the support for checksumming too (Add support for checksumming on heed3 #291)