RFC: Finalizing more naming conventions

This conventions RFC tweaks and finalizes a few long-running de facto
conventions, including capitalization/underscores, and the role of the `unwrap` method.

See [this RFC](https://github.com/rust-lang/rfcs/pull/328) for a competing proposal for `unwrap`.

[Rendered](https://github.com/aturon/rfcs/blob/finalizing-naming-conventions/text/0000-finalizing-naming-conventions.md)
I think we should consider `CamelCase` for constants, since `SCREAMING_SNAKE_CASE` is extremely abrasive to look at.