Add check for if a shard_amount is a power of two. (#186)

Signed-off-by: Leo Lu <luyuxuanleo@gmail.com>

src/lib.rs

@@ -250,6 +250,7 @@ impl<'a, K: 'a + Eq + Hash, V: 'a, S: BuildHasher + Clone> DashMap<K, V, S> {
         shard_amount: usize,
     ) -> Self {
         assert!(shard_amount > 0);
+        assert!(shard_amount.is_power_of_two());
 
         let shift = util::ptr_size_bits() - ncb(shard_amount);