[ruby-core:23049] Extending binding and set_trace_func in 1.9

It's been quiet in ruby-core.

Recently I've looking at introspection, tracing and writing a new debugger
for Ruby 1.9.

Here are two little extensions to some existing functions that I think will
help in tracing and writing a debugger.

1. set_trace_func().

Right now  set_trace_func() takes a function name. Internally though the C
code sets a trace mask to an event type to trigger (TRACE_MASK_ALL).  It is
helpful sometimes to pass a more selective mask. One possible approach is to
add this as an optional parameter.

In the attached patch this is done. However in the patch the optional
parameter is a Fixnum. I realize it might be nicer if the mask could be
instead set or a list of string event names. Since the change is in C code,
and I prefer to write Ruby, the way I'd suggest doing this is some outside
Ruby code that accomplishes this. Perhaps it could be added to tracer.rb

Not in the patch but something one may want to do is have a routine to
change the mask.

2. binding()

One of the things one wants to do in a debugger is to change the frame in
which to evaluate expressions. For this, ruby-debug adds Kernel#binding_n.
However, wouldn't it be nicer to just extend binding to allow an optional
level parameter (defaulting to 0) that returns a binding for the context
frame of the nth caller? Again this seems pretty simple to do in the
existing C code.