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Update TODO
Factor out a "command head" from eval.command
Fully expose eval.CommandHead; eval.builtinFunc -> BuiltinFunc
Let eval.Evaluator.{r R}esolveCommand return a CommandHead
eval.ioType -> .IOType

Consequently eval.Evaluator.ResolveCommand now fully exposes ..resolveCommand.
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Fix zeroing of Evaluator.filesToClose