Add `std` to search results

[jjant]

Searching "std" in the search bar should show the standard library but it doesn't: https://docs.rs/releases/search?query=std.

[syphar]

How would you imagine this looks like?
Currently docs.rs search is actually just calling out to the crates.io API and we return whatever the crates.io search returns.

Also, we have some redirects in place:

• docs.rs/std redirects to https://doc.rust-lang.org/stable/std/
• docs.rs/std::iterator redirects to https://doc.rust-lang.org/stable/std/?search=iterator

[jyn514]

Duplicate of #604