These AST nodes now have a flags field and then a bunch of optional
trailing objects. The end result is lower memory usage and consequently
better performance. This is part of an ongoing effort to reduce the
amount of memory parsed ASTs take up.
Running `zig fmt` on the std lib:
* cache-misses: 2,554,321 => 2,534,745
* instructions: 3,293,220,119 => 3,302,479,874
* peak memory: 74.0 MiB => 73.0 MiB
Holding the entire std lib AST in memory at the same time:
93.9 MiB => 88.5 MiB
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