gensym in Hy

Simple implementation of gensym in Hy.

Returns a new HySymbol.

Usable in macros like:

(defmacro nif [expr pos zero neg]
  (let [[g (gensym)]]
    `(let [[~g ~expr]]
       (cond [(pos? ~g) ~pos]
             [(zero? ~g) ~zero]
             [(neg? ~g) ~neg]))))

This addresses all the general comments about (gensym), and doesn't
try to implement "auto-gensym" yet. But clearly the macro approach
instead of the pre-processor approach (as described in the
letoverlambda (http://letoverlambda.com/index.cl/guest/chap3.html#sec_5)
is the way to go
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").
Merge branch 'master' into pr/374