Be more careful with as_int

The old version worked fine, but it felt a little sketchy the way we
were handling implicit conversion from a wrapped-around unsigned to a
smaller signed type.  (Note that if the signed type had been bigger, we
would silently get the wrong answer!)

au/code/au/utility/probable_primes.hh

@@ -189,7 +189,9 @@ struct LucasDParameter {
     uint64_t mag = 5u;
     bool is_neg = false;
 
-    friend constexpr int as_int(const LucasDParameter &D) { return D.is_neg ? -D.mag : D.mag; }
+    friend constexpr int as_int(const LucasDParameter &D) {
+        return bool_sign(!D.is_neg) * static_cast<int>(D.mag);
+    }
     friend constexpr void increment(LucasDParameter &D) {
         D.mag += 2u;
         D.is_neg = !D.is_neg;

au/code/au/utility/test/probable_primes_test.cc

@@ -222,6 +222,11 @@ std::vector<uint64_t> strong_lucas_pseudoprimes() {
             231703u, 243629u, 253259u, 268349u, 288919u, 313499u, 324899u};
 }
 
+TEST(LucasDParameter, CanConvertToInt) {
+    EXPECT_EQ(as_int(LucasDParameter{5u, false}), 5);
+    EXPECT_EQ(as_int(LucasDParameter{7u, true}), -7);
+}
+
 TEST(StrongLucas, AllPrimeNumbersAreProbablyPrime) {
     const auto primes = first_n_primes<3'000u>();
     for (const auto &p : primes) {