prepare for libdparse changes 1/2

[WebFreak001]

pairs with dlang-community/libdparse#491

reduces RAM usage of DCD by about a third (needs libdparse update first)

I would likely want to bump libdparse to 1.0.0 once we have the big AST nodes flattened and memory optimized.

[github-actions]

Thanks for your Pull Request and making D better!

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[WebFreak001]

moved to #740 which actually updates libdparse and will receive other changes too. (could have done it in here, but did it there now)