`{s with ...}` is now `{..., ..s}`, which more clearly expresses that the
result type is not necessarily equal to the type of `s` (in absence of an
expected type and a structure name, we still default to the type of `s`).
Multiple fallback sources can be given: `{..., ..s, ..t}` will fall back to
searching a field in `s`, then in `t`. The last component can also be `..`,
which will replace any missing fields with a placeholder.
The old notation will be removed in the future.
