If a chomp at [row][col] is legal, then (1) chomp all positions in the rectangle of positions with [row][col] being the upper-left-most and [rows-1][cols-1] being the lower-right-most, (2) change the current player from 1 to 2 or vice versa, and (3) return true.
Return the String representation of the board, such that each line is terminated with a newline, the first line consists of a space and the digits 0 through (cols-1)
and following lines consist of the row index digit followed by asterisks/spaces representing unchomped/chomped positions, respectively.