[m-dev.] for review: add `--debug' grade

[Zoltan Somogyi]

> > I also just added a new debugger command to print the stack trace, so
> > execution tracing now requires stack tracing.
> 
> I want to be able to use the debugger even on code that was not
> compiled in a .debug grade (e.g. I will just recompile the one module
> I'm interested in with `--trace all').

Sorry, I expressed myself badly. What I meant was that if you want the
debugger to give you all the information you ask for, you need stack tracing.
If you compile without stack tracing, you will get a partial stack trace
consisting of the initial top sequence of stack frames belonging to
modules compiled with tracing, and then a message saying the trace cannot
go further.

Zoltan.