Parse tuple expressions

[Kijewski]

Askama understands how to destructure tuples in let and match
statements, but it does not understand how to build a tuple.

This PR fixes this shortcoming.

[Kijewski]

I think this would resolve #607 quite nicely. It would compile the example to ↓, which is reasonably similar to what you would write by hand:

match &(
    self.client.can_post(&(self.post)),
    self.client.can_update(&(self.post)),
) {
    (false, false) => {
        writer.write_str("No!")?;
    }
    (can_post, can_update) => {
        writer.write_str("<ul>")?;
        if *(&(can_post) as &bool) {
            writer.write_str("<li>post</li>")?;
        }
        if *(&(can_update) as &bool) {
            writer.write_str("<li>update</li>")?;
        }
        writer.write_str("</ul>")?;
    }
}

[djc]

LGTM, thanks!

[Kijewski]

You're welcome :)