Expression parsing and evaluation improvements (wip)

This patch adds support for some simple expressions:

  penberg@titan:~/huron$ cat test/arithmetic.lisp | ./huron
  Huron, version 0.1
  > (+ 1 2)
  3
  > (- 1 2)
  -1
  > (* 5 5)
  25
  > (/ 5 5)
  1
  >
  Leaving Huron.

Signed-off-by: Pekka Enberg <penberg@kernel.org>