Remove all instances of static_cast<size_t>(1).

In preparation for removing magic numbers, simplify code that is
computing masks with a single bit set.

src/mem/allocstats.h

@@ -117,7 +117,7 @@ namespace snmalloc
     static constexpr size_t BUCKETS = 1 << BUCKETS_BITS;
     static constexpr size_t TOTAL_BUCKETS =
       bits::to_exp_mant_const<BUCKETS_BITS>(
-        (static_cast<size_t>(1) << (bits::ADDRESS_BITS - 1)));
+        bits::one_at_bit(bits::ADDRESS_BITS - 1));
 
     Stats sizeclass[N];
     Stats large[LARGE_N];