Infrastructure: Switch testing over to Autotest.

Currently mLib testing is a hopeless mess.  Maybe this will help sort it
out.

Some minor fixing was applied to the data-structure tests:

  * assoc and sym needed fixing to avoid corrupting their internal
    table-size count.  There are now additional assertions that this
    internal count matches the hash table's own count.

  * darray needed fixing to report empty-array on `first' and `last'
    requests.