Add the race function

[rcardin]

Racing two suspendable functions, foo and bar, should have the following semantics:

1. If foo completes, then return the foo result and cancel the bar execution;
2. If bar completes, then return the bar result and cancel the foo execution;
3. If one of the two functions fails at first, wait for the result of the other. If it's an error, return an error (containing both errors?), and return the second result otherwise.