Replace the restrictive `&mut RwTxn` by less restrictive `&RwTxn`

[Kerollmops]

The PR fixes #189 by removing all of the &mut RwTxn requirements by simple &RwTxn. If you want to understand the new possibilities I advise you to look at the example showcase non-mutable-rwtxn.

Doing so allows very cool use cases like:

• being able to dump database entries into another one.
• write into a database with values from the same database.
• [ ] Should we also use a &RwTxn when creating databases?

let wtxn = env.write_txn()?;
for result in database1.iter(&wtxn)? {
    let (k, v) = result?;
    database2.put(&wtxn, k, v)?; // It now compiles and works perfectly 🎉
}