integral_constant: As per std: Add operator().

In the C++ Standard Library, `std::integral_constant` has `operator()`:

https://timsong-cpp.github.io/cppwp/meta.help

We hereby add `operator()` for the Boost version.

test/integral_constant_test.cpp

@@ -0,0 +1,28 @@
+// Copyright 2025 DoctorNoobingstoneIPresume
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/type_traits/integral_constant.hpp>
+#include <boost/type_traits/is_same.hpp>
+#include <boost/static_assert.hpp>
+
+int main ()
+{
+    // [2025-01-07] Adapted from example in https://en.cppreference.com/w/cpp/types/integral_constant ...
+
+    using two_t  = boost::integral_constant <int, 2>;
+    using four_t = boost::integral_constant <int, 4>;
+
+    BOOST_STATIC_ASSERT ((! boost::is_same <two_t, four_t>::value));
+    BOOST_STATIC_ASSERT ((two_t::value * 2 == four_t::value));
+    BOOST_STATIC_ASSERT ((two_t () << 1 == four_t ()));
+    BOOST_STATIC_ASSERT ((two_t () () << 1 == four_t () ()));
+
+    enum class E {e0, e1};
+    using c0 = boost::integral_constant <E, E::e0>;
+    using c1 = boost::integral_constant <E, E::e1>;
+    BOOST_STATIC_ASSERT ((c0::value != E::e1));
+    BOOST_STATIC_ASSERT ((c0 () == E::e0));
+    BOOST_STATIC_ASSERT ((boost::is_same <c1, c1>::value));
+}