This is a partial revert commit of 8f096226.

NODE layout:

  Before:

       | ARYPTN | FNDPTN | HSHPTN
    ---+--------+--------+-----------
    u1 | pconst | pconst | pconst
    u2 | unused | unused | pkwargs
    u3 | apinfo | fpinfo | pkwrestarg

  After:

       | ARYPTN | FNDPTN | HSHPTN
    ---+--------+--------+-----------
    u1 | imemo  | imemo  | pkwargs
    u2 | pconst | pconst | pconst
    u3 | apinfo | fpinfo | pkwrestarg