feat: Add utility for forward declaring codecs

Add a simple macro which forward declares the full specialization for a
`codec<T>`. These forward declarations add a lot of noise while being
very similar and C++ doesn't really offer the tools to avoid these.

sources.cmake

@@ -29,6 +29,7 @@ dplx_target_sources(deeppack
         dp/fwd
         dp/indefinite_range
         dp/layout_descriptor
+        dp/macros
         dp/object_def
         dp/tuple_def
 

src/dplx/dp/macros.hpp

@@ -0,0 +1,31 @@
+
+// Copyright Henrik Steffen Gaßmann 2021.
+//
+// Distributed under the Boost Software License, Version 1.0.
+//         (See accompanying file LICENSE or copy at
+//           https://www.boost.org/LICENSE_1_0.txt)
+
+#pragma once
+
+#include <dplx/dp/fwd.hpp>
+
+// NOLINTBEGIN(modernize-macro-to-enum)
+// NOLINTBEGIN(cppcoreguidelines-macro-usage)
+
+#define DPLX_DP_DECLARE_CODEC_SIMPLE(_fq_type)                                 \
+    template <>                                                                \
+    class dplx::dp::codec<_fq_type>                                            \
+    {                                                                          \
+        using value_type = _fq_type;                                           \
+                                                                               \
+    public:                                                                    \
+        static auto size_of(emit_context &ctx, _fq_type const &value) noexcept \
+                -> std::uint64_t;                                              \
+        static auto encode(emit_context &ctx, _fq_type const &value) noexcept  \
+                -> result<void>;                                               \
+        static auto decode(parse_context &ctx, _fq_type &outValue) noexcept    \
+                -> result<void>;                                               \
+    }
+
+// NOLINTEND(cppcoreguidelines-macro-usage)
+// NOLINTEND(modernize-macro-to-enum)

src/dplx/dp/macros.test.cpp

@@ -0,0 +1,24 @@
+
+// Copyright Henrik Steffen Gaßmann 2021.
+//
+// Distributed under the Boost Software License, Version 1.0.
+//         (See accompanying file LICENSE or copy at
+//           https://www.boost.org/LICENSE_1_0.txt)
+
+#include <dplx/dp/macros.hpp>
+
+namespace dp_tests
+{
+
+namespace
+{
+
+class custom_type
+{
+};
+
+} // namespace
+
+} // namespace dp_tests
+
+DPLX_DP_DECLARE_CODEC_SIMPLE(dp_tests::custom_type);