* format.scm: New file, brought in from SLIB, with the following

* format.scm: New file, brought in from SLIB, with the following
changes:
(format:format): If the first argument is the format string, stick
a #f on the front of it, so it is now a valid CL format argument
list.  This is easier than changing everyplace else (like the
error formatter) that expects it to be in CL form.  The other
clause which explicitly tests for this case is now dead code.
(format:format-work): Allow `@' and `:' in either order, as per
modern CL behavior.
(format:num->cardinal): Don't assume that an elseless if returns
'() when the condition is false.