[discuss] Support for reverse-execution

[Michael Snyder]

 > Would not it be better to have a single "reverse" command which
 > puts the debugger in "reverse" mode -- making step/stepi/next/nexti
 > go backward.
 >
 > This might be handier for UI -- after all "step" can be abbreviated
 > as 's', but reverse-step can't be abbreviated. This might be better
 > in GUI too -- instead of doubling the number of "step" commands
 > you'd have a single "gear" switch, that can be in either "forward"
 > or "reverse" position.
 >
 > - Volodya

I'm certainly not opposed to that, although I lean a bit more
toward the separate "analogue" commands.

Fortunately, command interface is easy to implement.
Maybe we will float several independant patches that
each implement a different CLI to the same functionality,
so we can try them all out and get a feel for them.