Missing tests for unfold #677
Description
As the title says.
Unfold is a sort-of in-dataframe .toDataFrame() conversion (https://kotlin.github.io/dataframe/unfold.html), but this behavior isn't tested anywhere, nor do we check it's consistent with our Iterable<>.toDataFrame() overloads.
Related to unfold, we should probably rename AnyCol.isPrimitive() and KClass<*>.isPrimitive() to isValueType() or something, because "primitive" has a different meaning in the JVM world.