Historically, `SimpleXMLIterator` had only implemented `Iterator` and
`RecursiveIterator` methods. This changed in 8.0, when iterator methods
were moved to `SimpleXMLElement`, and `SimpleXMLIterator` was made a
dummy class extending `SimpleXMLElement`.
Fixesvimeo/psalm#6305, in the sense that Psalm would no longer report
different errors depending on the runtime PHP version.
Changed the format of the historical callmap to be plain signature list,
as it isn't really a delta file and renamed the file to not be confused
with deltas.
Added a test to ensure entries in main callmap have history.
