Instead of being its own node, it's a struct inside FnProto.
Instead of FnProto having a SinglyLinkedList of ParamDecl nodes,
ParamDecls are appended directly in memory after the FnProto.
throughput: 72.2 MiB/s => 72.9 MiB/s
maxrss: 70 KB => 68 KB
Importantly, the API is improved as well since the data is arranged
linearly in memory.
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