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