This is a breaking change. A call to `Response::headers()` can be
replaced with `Response::headers().iter()`. A call to
`Response::header_values()` can be replaced with
`Response::headers().get()`.
Here's the idea: under the `Rocket` namespace should live things critical to
writing simple Rocket apps: Request, Response, Error, etc. Nothing should be
nested more than one level deep. Only items required for more complex things
(implementing uncommon traits, etc.) should be nested one level deep.
This commit is the first attempt at realizing this.
