Prior to this commit, codegen emitted tokens containing bare types like
'Result' and 'Box' as well as presumed imported variants such as 'None'
and 'Ok'. However, users are free to shadow these, and if they do, the
generated code will fail to compile, or worse, be incorrect. To avoid
this, this commit makes all references to these core types and imports
absolute.
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.
