[CN-exec] Fixes for INI demo

[rbanerjee20]

Fixes for demo at Big Specification workshop at Newton Institute:

• Implementation of rem in CN runtime library
• Proper implementation of mod in CN runtime library (the two operators are not exactly the same, particularly in the case of negative numbers)
• Fix for translation of CN if-then-else. Was causing a segmentation fault (stack overflow) in seq_of_buf CN function, where the recursive call to seq_of_buf was being evaluated each time, regardless of what the condition evaluated to.