Fixes#8341.
The `CustomValue::to_json()` function is an odd duck; it defaults to
returning `null`, and no `CustomValue` implementations override it to do
anything useful. I forgot to implement `to_json()` for `SQLiteDatabase`,
so `open foo.db | to json` was returning `null`.
To fix this, I've removed `CustomValue::to_json()` and now `to json`
will collect a `CustomValue` into a regular `Value` before doing a JSON
conversion.
* change between lazy and eager
* when expressions
* examples for aggregations
* more examples for agg
* examples for dataframes
* checked examples
* cargo fmt
