In vanilla arc, 'each is a macro that expands to code that determines how to
traverse a data structure. This is redundant, and violates the principle of
never using a macro where a function will do; a higher-order fn that took a data
structure and a traversal fn could do the same and would be more generic.
Moreover, such a function could be extended via redefinition to allow new data
structures to be traversed. However, even given such a function, 'each would
still be useful as a convenience macro, to avoid unnecessary use of 'fn.

This hack refactors 'each into a higher-order fn 'walk, which takes an object to
traverse and a function to apply to each part of it, and a simplified 'each
macro that just wraps 'walk. Example:

    arc> (walk '(1 2 3) prn)
    1
    2
    3
    nil
    arc> (let old walk
           (def walk (x f)
             (old (if (and (isa x 'int) (>= x 0)) (range 1 x) x)
                  f)))
    *** redefining walk
    #<procedure: walk>
    arc> (walk 3 prn)
    1
    2
    3
    nil