Properly define (and maybe formalise) Tasks

[robotlolita]

As mentioned in #177, Tasks are still pretty ad-hoc and experimental, so a lot of edge cases may remain when dealing with concurrent execution. We should try to give people better tools to understand Task-based code, reason about concurrency with them, debug concurrency problems in Task-based code, and, of course, get rid of any bugs that remains in the implementation.

To this end we'll need:

• A specification for Task's semantics;
• A way of identifying tasks and transformations, so people can figure out where things are failing (think of this as a stack-trace, but for task interactions);
• Some model checking for finding race bugs, and better tests in other areas -- the interaction of task transformation with cancellation wasn't well tested;

This work subsumes the one in #153