Introduce a Value Type for Enso objects

[radeusgd]

In the in-memory Table backend, we can store any Enso object in a Column. Currently, such a column gets a Mixed datatype.

To allow for more precise types, we want to introduce the following Value_Type variant:

    Enso_Object type=Any

If the type is set to some specific Enso Type, the column will ensure that only values of that type are stored within it.

• [ ] Add the Enso_Object variant.
• [ ] Ensure operations like Column.from_vector, map, zip can check that the results fit this value type (using the is_a check).
  • Figure out how to implement this efficiently, without too much Java-to-Enso callbacks.
• [ ] Allow the cast operation to convert a Mixed column to a specific type, if all values satisfy it.
• [ ] ? Allow auto_value_types to select a more specific type in a Mixed column if Meta.type_of all values in a given column (excluding Nothing probably) yields the same type.
• This value type is not meant to be supported by the DB backends.

[radeusgd]

? Allow auto_value_types to select a more specific type in a Mixed column if Meta.type_of all values in a given column (excluding Nothing probably) yields the same type.

cc: @jdunkerley do you think we want auto_value_types trying to guess the Enso_Object type? IMO this could be nice.

[JaroslavTulach]

Idea: Use Meta.type_of to identify Enso type. When there are different types, unify to Any?