abacus: bignum: fix order comparisons

This is a pretty big oversight...

src/bignum/bignum.cc

@@ -16,6 +16,10 @@ namespace {
 auto static constexpr BASE = 10;
 
 bool do_less_than(digits_type const& lhs, digits_type const& rhs) {
+    if (lhs.size() != rhs.size()) {
+        return lhs.size() < rhs.size();
+    }
+
     return std::lexicographical_compare(lhs.rbegin(), lhs.rend(), rhs.rbegin(),
                                         rhs.rend());
 }

tests/unit/bignum.cc

@@ -60,6 +60,14 @@ TEST(BigNum, comparisons) {
     EXPECT_GE(one, one);
 }
 
+TEST(BigNum, comparisons_digits) {
+    auto const nine = BigNum(9);
+    auto const ten = BigNum(10);
+
+    EXPECT_LT(nine, ten);
+    EXPECT_GT(ten, nine);
+}
+
 TEST(BigNum, unary) {
     auto const zero = BigNum(0);
     auto const one = BigNum(1);