Add a board_cnt object that tracks the overlaps

[jonadsimon]

Add a parallel board object that tracks the number of overlaps in each position

This allows us to easily enforce multiple other types of constraints e.g. distributional, per row/column overlaps, single-word non-overlaps, etc

[jonadsimon]

Added a moderately-efficient version in parameterized_board_generator_fixed_overlaps.mzn.

For input "couch" w/ packing constant 1.2:

1. orginal runtime: 1.9s
2. runtime with naive board_cnt implementation: 28.4s
3. runtime with optimized board_cnt implementation: 13.6s

So still ~7x slower to even just instantiate, but better than the naive version which was 15x slower

[jonadsimon]

Not worth it, slows things down too much, discard this avenue of inquiry