Actix-web: fix `cargo deadlinks` results

[robjtede]

I suggest that when fixing, migrating to intra-doc links.

Found invalid urls in struct.NamedFile.html:
    Linked file at path :sync_all does not exist!
    Linked file at path /Users/rob/.cargo/target-shared/doc/io/index.html does not exist!
Found invalid urls in http/struct.HeaderMap.html:
    Linked file at path http/struct.GetAll.html does not exist!
Found invalid urls in error/index.html:
    Linked file at path error/Sender does not exist!
    Linked file at path error/Receiver does not exist!
Found invalid urls in error/struct.Canceled.html:
    Linked file at path error/Sender does not exist!
    Linked file at path error/Receiver does not exist!
Found invalid urls in web/index.html:
    Linked file at path web/Sender does not exist!
    Linked file at path web/Receiver does not exist!
Found invalid urls in trait.FromRequest.html:
    Linked file at path struct.PayloadConfig.html does not exist!
    Linked file at path struct.JsonConfig.html does not exist!
Found invalid urls in web/struct.Canceled.html:
    Linked file at path web/Receiver does not exist!
    Linked file at path web/Sender does not exist!
Found invalid urls in http/struct.HeaderMap.html:
    Linked file at path http/struct.GetAll.html does not exist!
Found invalid urls in dev/struct.JsonBody.html:
    Linked file at path dev/struct.JsonConfig.html does not exist!
Found invalid urls in error/index.html:
    Linked file at path error/Sender does not exist!
    Linked file at path error/Receiver does not exist!
Found invalid urls in dev/trait.Service.html:
    Linked file at path dev/fn.fn_service.html does not exist!
Found invalid urls in error/struct.Canceled.html:
    Linked file at path error/Sender does not exist!
    Linked file at path error/Receiver does not exist!
Found invalid urls in http/struct.HeaderMap.html:
    Linked file at path http/struct.GetAll.html does not exist!

[realaravinth]

We are re-exporting Canceled from futures_channel::oneshot::Canceled and cargo doc is inserting documentation from over at futures_channel. The links in the documentation there is relative to their crate so the links generated, too, are trying to point to things relative within actix-web. Because of this reason, the following dead links can't be fixed:

    Linked file at path error/Receiver does not exist!
    Linked file at path error/Sender does not exist!

But weirdly, Receiver and Sender are linked perfectly to their parent crates at docs.rs! I'm not really sure what is happening there. Those links, unlike the ones generated locally, are absolute.

[jyn514]

https://github.com/poliorcetics/cargo-intraconv might be helpful for automating the switch.