Add trap for divide by zero in expression library Added a trap for division by zero in the expression library and adjusted lwasm to use it and report division by zero errors instead of crashing with a cryptic "arithmetic exception" or even more mysterious "floating point exception".

The lwbasic directory is an in-progress attempt to write a compiler for a
dialect of basic specifically targetted at the 6809 CPU. It is still
incomplete but is included in this distribution in case someone finds the
code useful. It is not built by default.