The AsyncListener API has been moved into the "tracing" module in order
to keep the process object free from unnecessary clutter.
Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
Add a new 'tracing' module with a v8 property that lets the user
register listeners for gc events. The listeners are invoked after
every garbage collection cycle with 'before' and 'after' statistics.
Useful for monitoring tools that want to keep track of memory usage.
