Todo is fundamentally just a trace and an error. The only reason we kept it as a separate element in the AST is for the formatter to work out whether it should format something back to a todo or something else.
However, this introduces redundancy in the code internally and makes the AIR more complicated than it needs to be. Both todo and errors can actually be represented as trace + errors, and we only need to record their preferred shape when parsing so that we can format them back to what's expected.
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