Compare returns: - 0 if this and that are equal, - -1 is this is smaller and - +1 is this is bigger.

Compare_ returns: - 0 if this and that are equal, - -1 is this is smaller and - +1 is this is bigger.

Compare_i returns: - 0 if this and that are equal, - -1 is this is smaller and - +1 is this is bigger.

Sort sorts the slice inplace and also returns it.