pub fn write_u24_le(n: u32, slice: &mut [u8]) {
    slice[0] = n as u8;
    slice[1] = (n >> 8) as u8;
    slice[2] = (n >> 16) as u8;
}
Write a u24 (given as a u32 where the most significant 8 bits are ignored)
in little endian format to the beginning of the given slice. This panics
if the slice has length less than 3.
src/bytes.rs