Improve the Reader and Writer creation

[Kerollmops]

This PR improves the creation of a Reader and Writer types.

[irevoire]

Shouldn’t we use our own Database type in our examples?

I already did it in this PR for the movies examples.

But basically, we could change this:

    let database = env
        .database_options()
        .flags(DatabaseFlags::INTEGER_KEY)
        .types::<KeyCodec, NodeCodec<Euclidean>>()
        .create(&mut wtxn)?;

For this:

    let database: Database<Euclidean> = env
        .database_options()
        .flags(DatabaseFlags::INTEGER_KEY)
        .types()
        .create(&mut wtxn)?;

If we don't want to force people to use the Database type, then maybe we should move the mandatory KeyCodec and NodeCodec outside of the internal module?

[irevoire]

If we want to follow the rust convention shouldn't we call this one « into_bytes » since it makes an allocation instead of returning a reference to its content?

From what I see, as_bytes and to_bytes returns references while into_bytes generally return something you can own.

[Kerollmops]

I propose that we use self instead of &self and use the to_bytes name. What do you think?
https://doc.rust-lang.org/std/primitive.u32.html#method.to_le_bytes