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)
In the past we've discussed how "foo-bar" is somewhat more aesthetically pleasing than "foo_bar" when looking at package names. I do think that this convention will have large ramifications on the contents of crates.io and cargo in general, so just wanna make sure we're sure about this!

If we go with snake_case, then we probably want to remove the quotes around `extern crate "foo" as bar` as they kinda only exist today to allow dashes.