With the transition toward parsing with `rust-peg` and away from
`combine`, we're not using some of the many helpers we built to
support our unusual `combine` usage. They can just go!
I elected to keep Tolstoy using `failure::Error`, because Tolstoy
looks rather more like a high-level application (and will continue to
do so for a while) than a production-ready mid- or low-level API.
