8591: Remove SyntaxRewriter usage in insert_use in favor of mutable syntax trees r=matklad a=Veykril
Unfortunately changing `insert_use` to not use `SyntaxRewriter` creates a lot of changes since so much relies on that. But on the other hand this should be the biggest usage of `SyntaxRewriter` I believe.
8638: Remove SyntaxRewriter::from_fn r=Veykril a=Veykril
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
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