set_trace_func emulation in Rubinius

[Rocky Bernstein]

The first release rbx-tracer, set_trace_func emulation  for Rubinius 1.2 or (1.2.1dev) is now on gemcutter.org

This is basically a distillation of the stepping code that is in the reference and trepanning debuggers with some small additional code to run the callback hook.

The idea of adding set_trace_func in Rubinius was not just that I think  set_trace_func should be there for compatibility with MRI/YARV/Pickaxe, but 
also because stepping in Rubinius is a bit unique. "step over", "step out",  and "step into" are handled largely outside of the kernel, in the Ruby 
callback code. 

Thanks to Evan Phoenix and Brian Ford for all the help and support in Rubinius for making this possible.