Created a file for universally used code

For lack of a better term, changed Index to nni (non-negative integer)
as the name index only fits a small piece of its intended usage.
nni is basically what I would like size_t to be - a non-negative
integral value that fits the word size on the processor.
Add check-status flags for intelligent move generation
Keep things simple for the first implementation
Set check status when a move is made