Summary:
- A field value was being elaborated more than once when there is
another field whose default value depends on it.
The new test `structure_default_value_issue.lean` exposes the problem.
- Better error message and localization at field type mismatches.
When there is field type mismatch, the error message contains the
field name, and the error is reported at the field position instead of
`{`.
- We add support for auto_param at structure instances `{...}`
See #1422
