core: major refactor, removing the Group struct from board.go
core: continuing the refactor, removing Group references in display.go
core: unit test updates for grouping-refactor
core: fix some remaining group stuff in display.go
core: fix a new group merging bug in board.go
core: fix a new singleton liberty update bug in board.go
core: include liberties in GroupString
core: fix a hashing bug introduced by the group refactor