3250 - rocketnia's idea for extensible macros

http://arclanguage.org/item?id=18050
3251 - stop hiding false positives in tests
3252 - correction for extensible macros

Finally figured out why 3250 was eval'ing too much.
3253

Relying on something in 052 to make 040 pass is just too long a
dependency.
3254
3255 - rollback 3250

Counter-example:

  # Simplified expansion of:
  #   mac (foo x)
  #     `(+ ,x 1)
  #
  #   mac (foo x) :case nil
  #     `(list ,car.x)

  mac (foo x)
    `(if nil
       ,`(list ,(car x))
       ,`(+ ,x 1))

  (foo 3)  # throws unnecessary error

http://arclanguage.org/item?id=18081