Refactor table.group_by to table.aggregate

[jdunkerley]

Pull Request Description

Following UX work move to table.aggregate function.

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the Scala, Java, and Rust style guides.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed: Enso GUI was tested when built using BOTH ./run dist and ./run watch.

[radeusgd]

Looks good to me.

I really like how the unit tests already show how this new design seems to be more versatile and readable, even though I still find it a bit peculiar :)

[radeusgd]

Oh I missed something important.

The unit tests are missing any examples when the grouping happens over keys of more than one field. I think this should be checked - at least for 2-3 fields in the grouping. Also would be good to have at least some tests where the key fields are not first on the list - I know looking at the implementation this probably won't break - but good to check such a case since it is allowed and supported - at least to catch any future regressions and act as documentation of intent.

[radeusgd]

If the key argument is unused, maybe worth just making it take Nothing?
Well, technically even, just making it:

add_row =
    ... the code ...

...

row_index = map.get_or_else key add_row

should work because this will be a no-argument function.