Newsched TCP

This series mostly focused on implementing the TcpStream and TcpListener types ([usage example](https://github.com/brson/rust/blob/io/src/libcore/rt/io/net/tcp.rs#L158)). It isn't done, but several tests are working. The error handling strategy employed here is still the one I described a few weeks ago. It has received strong critique and will be changing.

There's also a lot of interspersed cleanup and docs.

Note that coretest on incoming is experiencing intermittent segfaults (#6258) for unknown reasons. I don't *think* this branch makes the situation worse, but it may by virtue of adding more scheduler tests.