[FR] `map` should reuse unchanged columns from the previous dataset to avoid disk usage

### Feature request

Currently adding a new column with `map` will cause all the data in the dataset to be duplicated and stored/cached on the disk again. It should reuse unchanged columns.

### Motivation

This allows having datasets with different columns but sharing some basic columns. Currently, these datasets would become too expensive to store and one would need some kind of on-the-fly join; which also doesn't seem implemented.

### Your contribution

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