No structural lists

Here's an initial attempt at eliminating `mk-structural-list` in favor of using the actual `link` and `empty` constructors, by creating placeholder values in `runtime.rkt` that are filled in by `moorings.arr` once `link` and `empty` are defined.  All tests pass for me, though I don't know if I've completely cleaned up everything that can be cleaned up.  (E.g., builtins.keys is now an eta-expanded call to prim-keys; it probably can be simplified...) Also, I wanted to name the exported variable `p:mk-list`, but I can't use that name in Pyret and so cannot fill in the placeholder.  Is there a better way to do this?