The "task" chapter is a stub

[japaric]

Considering that concurrency is the main selling point of Rust.

These topics should also be covered:

• How to reap failing tasks
• Barrier
• Future

Consider merging the channels chapter into the task chapter.
Consider adding an example about unsafe fork join parallelism. (unsafe because there is no library support for it at the moment)

[mdinger]

This seems out of date and is probably invalid