Significant rewrite

- Simplify Number::operator/= to use more constexpr values, and fewer
  variations.
  - Most significantly, rounding up doesn't need more precision, it only
    needs to know if there's a remainder after the current precision work
    is done. Tracked similarly Guard::xbit_.
- Build a constexpr lookup array for powers of 10. Only a handful of
  values are used, but since it's built at compile time, and constexpr,
  unused values do not affect memory or performance.

src/test/basics/Number_test.cpp

@@ -50,11 +50,15 @@ class Number_test : public beast::unit_test::Suite
     {
         T p = 1;
         if (n >= 0)
+        {
             for (int i = 0; i < n; ++i)
                 p *= 10;
+        }
         else
+        {
             for (int i = 0; i < -n; ++i)
                 p /= 10;
+        }
         return p;
     }