Allocation rules:
* Use `as-objc-allocation' when creating a Cocoa object. When the
resulting reference becomes unreachable, the Cocoa object will be
releaset.
* Use `with-autorelease' in atomic mode around calls that autorelease
and where the release should take effect immediate. Do not create
an autorelease pool except in atomic mode.
* Other autoreleased objects may end up in the root pool installed by
"pool.rkt". The root pool is periodically destroyed and replaced;
call `queue-autorelease-flush' if you need to encurage replacement
of the pool.
