Move collapse-board-init to different file
Make collapse-board functional
Add and test place-tile
Rename place-tile.rkt file from place-tile-board.rkt
Add place-tile to board.rkt
Rename backup/board.rkt to backup/board-backup.rkt
Make struct un-mutable
Modify replace-neighbour
Backup imperative count-neighbour
Functional version of count-neighbour working.
Partial testing done