Remove the extension traits for Readers/Writers

[alexcrichton]

These methods are all excellent candidates for default methods, so there's no need to require extra imports of various traits. Additionally, this was able to remove all the weird underscores after the method names. Yay!

[huonw]

Couldn't rt::io::extensions now be empty/deleted? (Presumably moving the remaining content to more appropriate places?)

[alexcrichton]

I'm not entirely certain of the fate of rt::io::extensions. Right now it's a small dumping ground for conversion between things, I ended up just dumping the only used std::io functions into it. Some more thought should probably go into the organization of the module before removing it outright.

Do we want to keep the bytes iterator? Do we want to start dumping byte conversions in rt::io? I suppose they kinda belong there b/c all the other conversions are currently done there, but moving the iterator into the module seems a little odd.

[brson]

The fate of extensions can be decided later. There's probably no disagreement on this step.