Let's use rapidjson instead of roll-your-own json. Added helpers to keep allocations minimized.

src/serialization.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <stdint.h>
+
+// if only there was a standard library function for this
+template<size_t Len>
+inline size_t StringCopy(char (&dest)[Len], const char* src) {
+    if (!dest || !src || !Len) {
+        return 0;
+    }
+    size_t copied;
+    char* out = dest;
+    for (copied = 1; *src && copied < Len; ++copied) {
+        *out++ = *src++;
+    }
+    *out = 0;
+    return copied - 1;
+}
+
+struct DiscordRichPresence;
+
+size_t JsonWriteRichPresenceObj(char* dest, size_t maxLen, const DiscordRichPresence* presence);
+size_t JsonWriteHandshakeObj(char* dest, size_t maxLen, int version, const char* applicationId);