allow different behavior on undefs

[rjbs]

What happens when you get an undef value for (say) %s or %t? Right now, it does the normal Perlish thing of converting to "" or 0. It should be possible to change this behavior, giving at least these options:

• do what we do now, coerce
• throw a runtime error
• for strings, supply a placeholder to be used for undef

There is no case when a payload value can correctly be undefined right now, so we can throw as soon as we have the payload. Further, we should be able to pull, out of the format string, a mapping of parameter names to legal types. Then we could validate things much more usefully.

This todo is the result of https://rt.cpan.org/Ticket/Display.html?id=67851