Tidy up tests and docs:

* Add handler_ptr test and increase coverage
* Add test for prepare_buffer
* Move is_call_possible tests to a .cpp file
* Tidy up docs and declarations

test/core/is_call_possible.cpp

@@ -0,0 +1,56 @@
+//
+// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// 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)
+//
+
+// Test that header file is self-contained.
+#include <beast/core/detail/is_call_possible.hpp>
+
+namespace beast {
+namespace detail {
+namespace {
+
+struct is_call_possible_udt1
+{
+    void operator()(int) const;
+};
+
+struct is_call_possible_udt2
+{
+    int operator()(int) const;
+};
+
+struct is_call_possible_udt3
+{
+    int operator()(int);
+};
+
+#ifndef __INTELLISENSE__
+// VFALCO Fails to compile with Intellisense
+static_assert(is_call_possible<
+    is_call_possible_udt1, void(int)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt1, void(void)>::value, "");
+
+static_assert(is_call_possible<
+    is_call_possible_udt2, int(int)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt2, int(void)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt2, void(void)>::value, "");
+
+static_assert(is_call_possible<
+    is_call_possible_udt3, int(int)>::value, "");
+
+static_assert(! is_call_possible<
+    is_call_possible_udt3 const, int(int)>::value, "");
+#endif
+
+}
+} // detail
+} // beast