abacus: bignum: use IIFE

I think it looks cleaner to use Immediately Invoked Function Expressions
in such cases.

src/bignum/bignum.cc

@@ -82,12 +82,12 @@ digits_type do_addition(digits_type const& lhs, digits_type const& rhs) {
         ++it2;
     }
 
-    auto it = it1;
+    auto [it, end] = [=]() {
-    auto end = end1;
+        if (it1 != end1) {
-    if (it1 == end1) {
+            return std::make_pair(it1, end1);
-        it = it2;
+        }
-        end = end2;
+        return std::make_pair(it2, end2);
-    }
+    }();
 
     while (it != end) {
         int addition = *it + carry;