* 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