Turn on Rust lexing. Add a bigger sample

This attempts to turn on the pygments lexer for Rust.

Pygments added support for Rust in 0.6, so assuming that GitHub has deployed 0.6, this should start highlighting Rust code. Unfortunately, Rust has changed a lot since so the results will probably not be great. Pygments was most recently updated for Rust [in January](https://bitbucket.org/birkenfeld/pygments-main/commits/5127d7fd8d30308b83a8e7481965c1b906a6f157), with a grammar that is much closer to final. I am hopeful that, even without the updated lexer, the results will at least be better than not having highlighting.

This also adds a big Rust sample, taken from the core library, MIT/ASL2 licensed.

Per the suggestions in the README file I have not tested this, and am simply hoping the bots enjoy it.