Experimenting with more structured ways to handle command-line input/output in Rust

Implement `Iterator` for `OptionalSubtagsIterator`

Small change to directly implement Iterator rather than implementing next() itself, which should allow iterator functionality to be used (filter, map etc)

Created by  finchie  on March 5, 2024
HL6ZEJTNQGFQDFI6MJHKITGZHCXC3OYC2O7KRD42D36PEU5C5YVAC
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