Previously, an exception thrown from a fork was rethrown:
- a) when the call to fork() returns if the fork completes synchronously
- b) otherwise, at the next explicit call to Sync() if there is one
- c) otherwise, when the call to the enclosing Join coroutine returns

In the case of infinite fork loops, this meant exceptions might never be
propagated.

Now, when an exception is thrown from a fork it's always rethrown when
the call to the enclosing Join coroutine returns. The body of the Join
coroutine just stops executing as soon as possible once a fork signals
an exception. This will be at the call to fork() if the fork completes
synchronously, or at any ensuing call to Sync() or fork() otherwise.

Essentially once an exception is signaled from a parallel fork, the next
call to fork() behaves like Sync() instead of creating a fork, and once
all the parallel forks have completed, execution resumes directly at the
call to the enclosing Join coroutine, where the exception is rethrown.

Previously when forked tasks didn't finish synchronously, they were
added to a list of children tasks and destroyed at the next sync.

But this meant that in the case of an infinite loop spawning new forks,
such memory could grow unboundedly and never be reclaimed.

Fixing this also required changing how exceptions in forks are propagated.