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.
