Tidy up core sources:

The core headers are moved to their own directory (but remain in
the same namespace).

include/beast/core/bind_handler.hpp

@@ -0,0 +1,68 @@
+//
+// 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)
+//
+
+#ifndef BEAST_BIND_HANDLER_HPP
+#define BEAST_BIND_HANDLER_HPP
+
+#include <beast/core/handler_concepts.hpp>
+#include <beast/core/detail/bind_handler.hpp>
+#include <type_traits>
+#include <utility>
+
+namespace beast {
+
+/** Bind parameters to a completion handler, creating a wrapped handler.
+
+    This function creates a new handler which, when invoked with no
+    parameters, calls the original handler with the list of bound arguments.
+    The passed handler and arguments are forwarded into the returned handler,
+    which provides the same `io_service` execution guarantees as the original
+    handler.
+
+    Unlike `io_service::wrap`, the returned handler can be used in a
+    subsequent call to `io_service::post` instead of `io_service::dispatch`,
+    to ensure that the handler will not be invoked immediately by the
+    calling function.
+
+    Example:
+    @code
+    template<class AsyncReadStream, class ReadHandler>
+    void
+    do_cancel(AsyncReadStream& stream, ReadHandler&& handler)
+    {
+        stream.get_io_service().post(
+            bind_handler(std::forward<ReadHandler>(handler),
+                boost::asio::error::operation_aborted, 0));
+    }
+    @endcode
+
+    @param handler The handler to wrap.
+
+    @param args A list of arguments to bind to the handler. The
+    arguments are forwarded into the returned object.
+*/
+template<class CompletionHandler, class... Args>
+#if GENERATING_DOCS
+implementation_defined
+#else
+detail::bound_handler<
+    typename std::decay<CompletionHandler>::type, Args...>
+#endif
+bind_handler(CompletionHandler&& handler, Args&&... args)
+{
+    static_assert(is_CompletionHandler<
+        CompletionHandler, void(Args...)>::value,
+            "CompletionHandler requirements not met");
+    return detail::bound_handler<typename std::decay<
+        CompletionHandler>::type, Args...>(std::forward<
+            CompletionHandler>(handler),
+                std::forward<Args>(args)...);
+}
+
+} // beast
+
+#endif