Option to stitch multiple snippets together into a single test

[Vlad-Shcherbina]

Some documentation styles intersperse code snippets with explanations.

Example

First, you need to define a function:

fn hw() {
    println!("hello world");
}

And then you call it:

fn main() {
    hw();
}

---

The second snippet won't compile without the first one, and it does not really make sense to put hw() definition in a template.

It would be nice if this could be expressed in the following way:

First, you need to define a function:


'''rust,sk-part-of-fn-tutorial
fn hw() {
    println!("hello world");
}
'''

And then you call it:

'''rust,sk-part-of-fn-tutorial
fn main() {
    hw();
}
'''