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)
We could make them optional, that is, `extern crate foo as bar` and `extern crate "foo" as bar`. (If we remove support for strings, then we essentially require that crates have valid identifiers as names, and so should probably have the compiler enforce this?)