despair, debugging, and the repl

[Andy Wingo]

Greets,

I have on my list of things to do for 2.0 a line that says "REPL docs",
by which I take it to mean that I should be updating the docs in the
manual for the command-line user experience.

I think I've tried to tackle this a couple of times before, and I am
remembering why I stopped:

  * There is a fair amount of old code here (scm-style-repl)

  * There is new code that does the same thing (the (system repl repl)
    module)

  * The old repl code is crufty but surely we have to support some of
    its interfaces (?)

  * Neil's debugger code doesn't work any more but is prettier than
    mine, and did stepping which mine doesn't yet

  * Neil's debugger (and Guile pre-1.9, generally) worked on the
    expression level rather than the function call level, so it could
    give the user nicer information in many cases.

The cumulative effect of all this stilled me back to inaction in the
past, but we should fix this now, I think.

So, to that end, then, here's a preliminary plan of action:

  * Deprecate all of the old repl code, keeping compatibility shims for
    the useful interfaces perhaps, but trying to move the REPL
    implementation entirely out to modules.

  * Remove all of the old debugger code. This pains me somewhat, but we
    have to move forward here.

  * Deprecate the-last-stack fluid -- besides being hard to reason
    about, it doesn't even work all that well.

  * Document the new REPL.

I hope to finish this work within a week or so. Comments welcome!

Andy
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