Previously, a Request's only lifetime parameter referred to itself. This
causes many issues and is simply wrong. Instead, use `transmute` to make
the lifetime `static`. As long the contents inside Request don't move or
change, the references are valid. We keep the lifetime as a phantom in
`Request` for future use.
* All From* trait methods are now named like the trait.
* All From* traits have an associated Error type.
* Document all of the `form` module.
* Add codegen tests for auto-derived forms.
* The param parsing traits now live under Request.
* Add content-type responsers for JSON, HTML, and plain text.
* Use content-type responders in content_type example.
* Conditionally create Request `from` HypRequest.
* Clean-up dispatching and handling in main rocket.
* Change Level enum to Logging Level and reexport.
* Allow users to set logging level before launch.
* Fix content_type example error handling.
* Percent decode params when user requests `String`.