[integer] Refine arithmetic functions of BigUInt

[forfudan]

Refine the arithmetic functions for BigUInt

1. Add subtract_inplace()
2. Optimize the cases when the second argument is one-word long.
3. Refine the estimation of quotient for division using 3-by-2 division.
4. Update the function is_one() and is_two.
5. Remove unused function floor_divide_partition().