Add tests for `unfold`

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.

Start by locating the `unfold` implementation and the existing `Iterable<>.toDataFrame()` overloads, then read the linked unfold documentation for the intended behavior. Add tests covering unfold and consistency with those overloads; treat the possible `AnyCol.isPrimitive()` and `KClass<*>.isPrimitive()` rename as a separate scope decision.

Written by the indexing model from the issue text.