Adding automatic gensym macro

Adding to the manual gensym for macros are 2 new
macros, but very literal from the CL in
letoverlambda.

The first is the (with-gensyms ...) macro that
can generate a small set of syms for a macro. Works
something like:

(defmacro adder2 [A B]
  (with-gensyms [a b]
    `(let [[~a ~A] [~b ~B]]
       (+ ~a ~b))))

and ~a and ~b will be replaced with (gensym "a") and
(gensym "b") respectively.

Then the final macro is a new defmacro that will automatically
replace symbols prefaced with "g!" with a new gensym based on the
rest of the symbol. So in this final version of 'nif':

(defmacro/g! nif4 (expr pos zero neg)
  `(let [[~g!result ~expr]]
     (cond [(pos? ~g!result) ~pos]
           [(zero? ~g!result) ~zero]
           [(neg? ~g!result) ~neg])))

all uses of ~g!result will be replaced with (gensym "result").