use new IO crate

[zonyitoo]

Make JSON module compatible with the new IO crate.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[BurntSushi]

This is great, and it's nice to see that it didn't require that many changes!

I will beg the powers that be not to merge this yet though. I don't think the world is ready for std::io yet (there are still some really important things missing that are scheduled for this week), and rustc-serialize is a core library that tons of crates depend on.

[zonyitoo]

I have spent a whole day on migrating my small project to be compatible with the new IO crate. ;)

[alexcrichton]

While std::io may not be fully fleshed out yet I would like to make progress in allowing dependencies to update to using std::io wherever possible. I'm going to publish this as a new series of releases on crates.io so you can still opt-in to the std::old_io behavior. We can also perhaps maintain a branch for a bit with std::old_io as well.

[alexcrichton]

I've created an old-io branch we can use to maintain the 0.2.x series temporarily.