Add if let expressions example

[johannhof]

Maybe it's me, but I really needed an example to understand if let and refutable statements.
Playpen: http://is.gd/mjX3Gf

Let me know if the variable names are too, uh, culinary.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[johannhof]

r? @steveklabnik

[steveklabnik]

four spaces

[steveklabnik]

(for indentation, sorry)

[johannhof]

@steveklabnik changed to This body will be skipped because the pattern is refuted because I agree that "not execute" implies some sort of error. Also fixed the spaces. 🐠

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 6a19046 has been approved by steveklabnik

[steveklabnik]

Thanks so much! make sure to chime in over at #25196