Conditions and debugger support

[Brian T Rice]

We're cranking out a lot of changes recently. Here's condition/
exception-handling support, all in Slate, improved a bit to work in the
Smalltalk-ish frame of mind from what good bits that Dylan and Common Lisp
had.

See src/condition.slate and src/debugger.slate for the source (not
rocket science), and http://slate.tunes.org/progman/node21.html for some
documentation (it's a tad out of date, handlerCase: is now handlingCases:,
and there are a few more selectors worth knowing already).

The debugger right now is simply a way to get a backtrace shown, with
source file and line number information (the interpreter was already
collecting this info), and to select a restart.

Not everything is ready to ship, yet, though, but I'm making sure to fill
out the error-handling parts of the SUnit port (src/test.slate) before 0.2
goes.

There is also support for image-saving under CLISP, and this is
interacting a little oddly with the condition system, but the remaining
glitch or two should be fixed really soon.

Enjoy!

-- 
Brian T. Rice
LOGOS Research and Development
http://tunes.org/~water/