reuse: add example from tutorial

[nathantypanski]

This adds the example provided in the standard Rust tutorial, with a short description, to provide an example of using traits for reuse.

Perhaps we should come up with a better example than this Printable one, but I couldn't think of anything short enough that is actually superior for understanding the concept.

Discussion could still use elaboration, but I figure having anything on this page is better than before.

[aturon]

Thanks for another contribution! This is a good start, although we ultimately need to give some guidance on when reuse is a good idea. (I haven't thought deeply enough about it to have one, yet.)

I'm happy to merge this if you drop the final sentence about implementing common traits, which doesn't seem highly related.

[nathantypanski]

Updated as noted. I agree the last sentence was not very related.

And, yeah, I realize we need more discussion about sharing implementations and when it's smart to do so. This is just the mindless drudge work of "get something out there and find a simple example".

[aturon]

@nathantypanski Thanks! And I really appreciate all the help!