Used for conversion between different representations of bit array. Returns (n+(div-1))/div, rearranged to avoid arithmetic overflow. For example, in the bit to int case, the straightforward calc would be (n+31)/32, but that would cause overflow. So instead it's rearranged to ((n-1)/32) + 1, with special casing for 0. Usage: GetArrayLength(77, BitsPerInt32): returns how many ints must be allocated to store 77 bits.