LibIPC+IPCCompiler: Add IPC::Decoder, let classes decode themselves

This shaves ~5 seconds off of a full build, not too bad. Also it just
seems nicer to push this logic out to classes. It could be better but
it's a start. :^)

Libraries/LibIPC/Decoder.h

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <LibIPC/Message.h>
+
+namespace IPC {
+
+template<typename T>
+bool decode(BufferStream&, T&)
+{
+    return false;
+}
+
+class Decoder {
+public:
+    explicit Decoder(BufferStream& stream)
+        : m_stream(stream)
+    {
+    }
+
+    bool decode(bool&);
+    bool decode(u8&);
+    bool decode(u16&);
+    bool decode(u32&);
+    bool decode(u64&);
+    bool decode(i8&);
+    bool decode(i16&);
+    bool decode(i32&);
+    bool decode(i64&);
+    bool decode(float&);
+    bool decode(String&);
+
+    template<typename T>
+    bool decode(T& value)
+    {
+        return IPC::decode(m_stream, value);
+    }
+
+private:
+    BufferStream& m_stream;
+};
+
+}