Far from ready, and does currently not even compile.

Doesn't make it better as long as seq is not a builtin function.

It doesn't break existing functionality at least, but before it can
work, the general revamp of the lambda list processing must be done.
Which very likely means the keyword parameter binding has a different
technical base to run on, so it will have to be adapted to that *at
least*.

Also, needed most prominently here, but also elsewhere, a lob.VarMap
type for symbol => value mappings.

Decided by now: the keywords will be a map in the function definition,
the keywords the keys (surprise, surprise) with the defaults as
values. Maybe I'll need them as a list, too, in order to fill in
the missing one; I don't know yet. Until then, they will be printed
in the function ReaderString() in unpredictable order.

And in the course of doing that, finally fixed ingering bugs in the
:keyword processing of open and with-open-file. The latter, by the
way, should be replaced by a macro soon now that we have unwind-protect
and friends.

Also, a better error message in testing when using test-err with
not a regexp as the expected value.

Unit tests greatly improved, regtests still not done (parse-integer).

substring kind of fell out of parse-integer, because the latter
needed a clean substring.