Hershey fonts

Toy implementation of the Hershey font format in Rust. The reader and viewer components are separate, so feel free to pull out the reader for your own stuff. I'm not 100% confident with my parser writing skills, but it parsed all of the default fonts without erroring, so...

Also, check out hershey.txt to see some history and the usage terms of the default fonts I've copied over here. Thank you!