Rust doc comments

[fpieper]

At least the avl_tree.rs should have Rust doc comments. Example: https://doc.rust-lang.org/rust-by-example/meta/doc.html

Something along the lines of this:

/// Returns a person with the name given them
///
/// # Arguments
///
/// * `name` - A string slice that holds the name of the person
///
/// # Returns
///
/// * `person` - Person for the given name.

[flix59]

Added