std::io should re-export more sub-items

[alexcrichton]

It's a little annoying to import buffered::BufferedReader and mem::MemWriter. It'd be really nice if the imports were std::io::BufferedReader and std::io::MemWriter.

If possible, I would also like to make both the mem/buffered and whatever other modules are re-exported private.

[alexcrichton]

Nominating, if we decide to make submodules private this is a backwards-compatibility issue.

[sfackler]

If the standard library is going to lean more heavily on reexports, it'd be nice to figure out some way of better presenting the documentation for the reexported items. Currently, I think it ends up living in the module it was defined in, even if that module isn't reachable. This ends up being confusing since you see documentation at a path that you can't import.

[pnkfelix]

Accepted for 1.0, P-backcompat-libs.

[brson]

We're exporting a lot now. What's left?

[alexcrichton]

Many of the utilities are currently being reexported, but very few of the I/O primitives other than File are being exported. I would expect io::TcpStream and io::UpdSocket to both be types at the top-level, along with other I/O primitives in networking and such.

This issue is becoming quite vague, however, so I think that I'm going to close this in favor of more specific issues targeting the modules that I still have in mind.