Refactor the unit tests and examples

In particular, merge some unit tests and examples to reduce compilation
times.

test/ext/std/integer_sequence/iterable.cpp

@@ -0,0 +1,35 @@
+/*
+@copyright Louis Dionne 2014
+Distributed under the Boost Software License, Version 1.0.
+(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+ */
+
+#include <boost/hana/ext/std/integer_sequence.hpp>
+
+#include <boost/hana/detail/assert.hpp>
+
+#include <utility>
+using namespace boost::hana;
+
+
+int main() {
+    // head
+    {
+        BOOST_HANA_CONSTEXPR_ASSERT(head(std::index_sequence<0>{}) == 0);
+        BOOST_HANA_CONSTEXPR_ASSERT(head(std::index_sequence<0, 1>{}) == 0);
+    }
+
+    // is_empty
+    {
+        BOOST_HANA_CONSTANT_ASSERT(is_empty(std::index_sequence<>{}));
+        BOOST_HANA_CONSTANT_ASSERT(!is_empty(std::index_sequence<0>{}));
+        BOOST_HANA_CONSTANT_ASSERT(!is_empty(std::index_sequence<1>{}));
+    }
+
+    // tail
+    {
+        BOOST_HANA_CONSTANT_ASSERT(equal(tail(std::index_sequence<0>{}), std::index_sequence<>{}));
+        BOOST_HANA_CONSTANT_ASSERT(equal(tail(std::index_sequence<0, 1>{}), std::index_sequence<1>{}));
+        BOOST_HANA_CONSTANT_ASSERT(equal(tail(std::index_sequence<0, 1, 2>{}), std::index_sequence<1, 2>{}));
+    }
+}