Iterator functions are now allowed to have a close argument.
If they do, they will be called with a close = TRUE value
when iteration is terminated early. This gives the opportunity
to clean up resources.

Generators now run on.exit() expressions when they are closed.

Iterators managed by coro::loop() and by generator for loops
are now cleaned up when terminated early, either because of an
error or because of a break (#52).

Implicit and explicit return values of generators are no longer
yielded. This is consistent with Javascript and Python and simplifies
certain idioms (#51).

Generators and async functions assigned in namespaces no
longer produce R CMD check notes about visible bindings (#40).