Further leverage div_ceil (#689)

Followup to #688 which uses `div_ceil` in feature-gated code, which was
missed before

src/uint/boxed/div.rs

@@ -50,7 +50,7 @@ impl BoxedUint {
     ///
     /// Variable-time with respect to `rhs`
     pub fn div_rem_vartime(&self, rhs: &NonZero<Self>) -> (Self, Self) {
-        let yc = ((rhs.0.bits_vartime() + Limb::BITS - 1) / Limb::BITS) as usize;
+        let yc = rhs.0.bits_vartime().div_ceil(Limb::BITS) as usize;
 
         match yc {
             0 => panic!("zero divisor"),
@@ -75,7 +75,7 @@ impl BoxedUint {
     ///
     /// Variable-time with respect to `rhs`.
     pub fn rem_vartime(&self, rhs: &NonZero<Self>) -> Self {
-        let yc = ((rhs.0.bits_vartime() + Limb::BITS - 1) / Limb::BITS) as usize;
+        let yc = rhs.0.bits_vartime().div_ceil(Limb::BITS) as usize;
 
         match yc {
             0 => panic!("zero divisor"),

src/uint/encoding.rs

@@ -532,7 +532,7 @@ pub(crate) fn radix_encode_limbs_mut_to_string(radix: u32, limbs: &mut [Limb]) -
     let mut out;
     if radix.is_power_of_two() {
         let bits = radix.trailing_zeros() as usize;
-        let size = (limbs.len() * Limb::BITS as usize + bits - 1) / bits;
+        let size = (limbs.len() * Limb::BITS as usize).div_ceil(bits);
         out = vec![0u8; size];
         radix_encode_limbs_by_shifting(radix, limbs, &mut out[..]);
     } else {