But only non-validation error paths. When validating we do expect it to
maybe fail, so we don't want to generate cores for validation.
Validation is in fact a de-serialization pass with some additional
checks. To be able to keep reusing the same code for de-serialization
and validation just with different error handling, introduce a
`strict_mode` flag that can be passed to `composite::iterator`
constructor. When in strict mode (the default) the iterator will convert
any `marshal_exception` thrown during the de-serialization to
`on_internal_error()`.
We don't want anybody to use the iterator in non-strict mode, besides
validation, so the iterator constructors are made private. This is
standard practice for iterators anyway.
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