BranchAndBound: Only reduce as necessary in right branch
BranchAndBound: Only reduce as necessary in right branch

We don't need to call the row_reduce! and col_reduce! on the entire
matrix, since we know exactly which row/column might have changed.

30.6 -> 26 seconds

petertseng/adventofcode-rb-2016,petertseng/adventofcode-rb-2016