`unfold` / `toDataFrame` behavior for `Maps`

[Jolanrensen]

Could we make it so that kotlin.Map<String, T> is recognized and unfolded to a KeyValueProperty/NamedValuePair by default?

Currently, since toDataFrame { properties() } is the default, they are unfolded like:

which is not very helpful.

[Jolanrensen]

@AndreiKingsley
Let's make Map a value type instead of unpacking it. It becomes very difficult to go from List -> DataFrame -> List if we start doing something special for certain types like Maps.

We can still create a List<Map<String, T>>.toDataFrame() and Map<String, T>.toDataRow() overload.