These SuffixOp nodes have their own ast.Node tags now:
* ArrayInitializer
* ArrayInitializerDot
* StructInitializer
* StructInitializerDot
Their sub-expression lists are general-purpose-allocator allocated
and then copied into the arena after completion of parsing.
throughput: 72.9 MiB/s => 74.4 MiB/s
maxrss: 68 KB => 72 KB
The API is also nicer since the sub expression lists are now flat arrays
instead of singly linked lists.
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