Add support for dtrace/SystemTap

Similar to the probes that will be added to glib,
this commit adds basic infrastructure for probes, and
a *very* limited set of probe points.

The current probe points just track proxies for GObjects; other
ideas for useful probes would be function invocations, boxed/structs,
etc.

https://bugzilla.gnome.org/show_bug.cgi?id=614619