New PCL symbol-macrolet walker

[Trent Lange <lange@CS.UCLA.EDU>]

Hi --

I've finally gotten a chance to pick up your new PCL walk.lisp file
from CMU 16c to handle symbol-macrolet, which I plan on including in
my extensions/optimizations to Rich Harris' March 92 PCL that I'm
about to send on to him and/or release on xerox.com.

It seems to work well, except for one problem:  it drops out part of
the variable list on macrolets.  For example:

(walk-form
  '(macrolet ((foo ((x y) z) `(list ,x ,y ,z)) (foo (a b) c))))

Evaluates to:

'(MACROLET ((FOO (X Y) `(LIST ,X ,Y ,Z)) (FOO (A B) C)))

I don't know if this happens anywhere else.  On the other hand, it seemed
to work fine in the old walker.

If you could take a look at this, it would be great.

Thanks,

- Trent