Issue title: Fix Rust logo example
 Issue body: For the past several years, we've had [an example that draws the Rust logo](https://github.com/sunjay/turtle/blob/master/examples/rust.rs) using this crate:

![rust logo turtle](https://turtle.rs/assets/images/rust.gif)

As part of the new architecture in #173, we made a breaking change that resulted in the turtle using a circular brush instead of a rectangular brush. This resulted in the Rust logo example looking much worse than it used to:

![image](https://user-images.githubusercontent.com/530939/82846235-cfc0a700-9eb5-11ea-8360-46cfdd892eb0.png)

We can fix this by updating the example to draw a filled outline of the R instead of trying to draw it by increasing the pen thickness. This would essentially end up being the same technique used to draw the gear and the holes on the gear to draw the R. It will take some creativity and experimentation, but I definitely think it can be done!

Anyone is welcome to try this out! I would love to get this example working again!
 Comments: 
 Comment 0: This seems like a good one to get started learning turtle from a user perspective. I will give this a go.
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