## Summary
This PR removes the `Iterator::chain(...)` sequence in
`RuleCodePrefix::iter()` with `Vec::expand` to avoid an
overlong-recursive types.
The existing `RuleCodePrefix::iter` method chains all rule group
iterators together. This leads to very long recursive types
`Chain<Map<Chain<Map<Chain<Map.....>>>>` (proportional to the number of
rule groups).
This PR rewrites the macro to use `Vec::extend` instead, which removes
the long recursive type (at the cost of introducing a potential
allocation).
## Alternatives
An alternative would be to use a stack allocated array by unrolling the
`Linter::iter` methods (generated by `EnumIter`).
I don't think it's worth the extra complexity, considering that
`RuleCodePrefix::iter` isn't a hot function.
## Test Plan
`cargo test`
31db1b6e16