Make it possible to search for docs across crates

[derekchiang]

Currently libextra is being split up into separate crates. This however has the side effect that the documentation is now also scattering around, making it more difficult to find the code you want.

So, should we implement a unified search bar somehow? It could be something similar to this, or just something built on top of the current rustdoc interface that searches across all crates.

[lifthrasiir]

This is largely addressed by #12946 now. I'm not sure if this issue also includes a unified search bar in the main documentation page however.

[derekchiang]

Yeah, I think what we have is good enough for now.

[alexcrichton]

Closing, sounds like a more up-to-date issue should be opened with any new updates.