This is required to be able to do anything useful with the body in the
outgoing response. Request fairings do not appear to need to be async
as everything on Data that returns a future moves self and on_request only
gets &Data, but the same change in this commit should work for on_request
if desired.
This commit introduces the sealed `UriPart` marker trait as well as the
implementing `Path` and `Query` marker types, allowing for parts of a
URI to be distinguished at the type level. Consequently, `UriDisplay`
has been parameterized with `P: UriPart`, creating `UriDisplay<Path>`
and `UriDisplay<Query>`. The effect of this change is improved type
safely for URI rendering as well as the ability to omit rendering values
in query parts via `Option` and `Result`.
The `UriDisplay` derive was replaced by `UriDisplayQuery` and
`UriDisplayPath` which derive implementations for `UriDisplay<Path>`
and `UriDisplay<Query>`, respectively.
This commit also works around a rustdoc visibility issue by creating a
hidden `http::private` module.
Finally, this commit also removes the now vestigial use of the
`rustc_private` feature in codegen.
Fixes#827.