automatically skip `Base.iterate`

[MilesCranmer]

Base.iterate is unstable by design. I don't agree with the design choice but since it always is unstable, even for user code, there's no reason to flag it.

This also gives the user an alternative to the use of @unstable. They can add functions to ignore_functions. Though I think using @unstable is still better so I didn't export ignore_functions yet.

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Also adds various tests to increase coverage

[coveralls]

coverage: 100.0% (+3.3%) from 96.691% when pulling d29f608d62dc3fc86337044d008eaf574348f76c on ignore-iterate into 569aac2fc3a904aa6e499a6a0b406aaa3a3ed849 on main.