Refactor the code of Bitmap::new

[HaoYang670]

Expected behavior
A clear and concise description of what you expected to happen.

    pub fn new(num_bits: usize) -> Self {
        let num_bytes = num_bits / 8 + if num_bits % 8 > 0 { 1 } else { 0 };
        let r = num_bytes % 64;
        let len = if r == 0 {
            num_bytes
        } else {
            num_bytes + 64 - r
        };
        Bitmap {
            bits: Buffer::from(&vec![0xFF; len]),
        }
    }

We can use bit_util::ceil and bit_util::round_upto_multiple_of_64 to refactor the code.

Additional context
Notice when working on #1230

[HaoYang670]

Hope we could get a little performance improvement from it because faster arithmetical calculation is used:

pub fn round_upto_power_of_2(num: usize, factor: usize) -> usize {
    debug_assert!(factor > 0 && (factor & (factor - 1)) == 0);
    (num + (factor - 1)) & !(factor - 1)
}