On to primitives

* @expose now understands a list of names, the first one being the
  primary
* zero?, odd?, even?, negative?, positive? for numbers
* call-with-current-contiuation as alias for call/cc
* Very naive append implementation (needs continuation refactoring)

* Tests: the various run* functions by default now do _NOT_ use the
  stdlib anymore (works nonetheless)
* Tests: pseudo-doctest now can do some 'setup' with '!':

    def test_foo(doctest):
        """
        ! (require racket/private/map)
        > (let ([x 1])
            (for-each (lambda (a b c)
                         (set! x (+ x (* a b c))))
                       '(1 2 3) '(4 5 6) '(7 8 9))
             x)
        271
        """
        assert doctest

* one test for for-each
* for-each and append need the var-arg versions