Inlined blanks are now returned as blanks not at zeros.
Removed redundant BlankCell module
Stop replacing 0^X and 0/X with zero. If X is zero, then an error is the correct answer.
Add some better error reporting to excel_to_x
Now replaces IF statements at compile time if the condition can be evaluated
Improved the number of cases where SUM is partially eliminated
Tiny tweak to simplify_arithmetic to reduce stack size