Various stuff:

  * DEFREADTABLE does _not_ have compile-toplevel effects anymore.
    
    This means you cannot use a readtable defined via DEFREADTABLE
    in the same file it's defined. Use an explicit EVAL-WHEN if you
    want to achieve that.

    Advantage: It's not necessary anymore to wrap reader-macro function
    definitions in EVAL-WHEN.

  * Add restartable conditions READTABLE-DOES-NOT-EXIST, and
    READTABLE-DOES-ALREADY-EXIST.