fix(numerics): add missing numeric_traits<float>::zero()

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/tests/util/numerics/numeric_traits.cpp

@@ -33,6 +33,7 @@ static void tst1() {
     tst_num(mpfp(0.0, 512));
     tst_num(mpbq(3));
     tst_num(1.0);
+    tst_num(static_cast<float>(1.0));
 }
 
 int main() {

src/util/numerics/float.cpp

@@ -25,7 +25,7 @@ void float_ceil(float & v)  { v = std::ceil(v); }
 void float_floor(float & v) { v = std::floor(v); }
 
 static float g_zero = 0.0;
-float const & zero() {
+float const & numeric_traits<float>::zero() {
     return g_zero;
 }
 };