The counters are unsigned, so that they can overflow. (Is that really
necessary though?) In any case try_pop can decrement a counter, leading
to a situation where you can think you have (size_t)-1 elements; not
good. Instead when computing the queue size, use a signed value.
Limits total queue size to half the unsigned space; fine.
The collector now has an abstract interface onto the embedder. The
embedder has to supply some functionality, such as tracing and
forwarding. This is a pretty big change in terms of lines but it's
supposed to have no functional or performance change.
