Add key functions to Table to make it act as [Column]

[radeusgd]

Pull Request Description

Implements https://www.pivotaltracker.com/story/show/181370836

Important Notes

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the
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  and
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• 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 ide build and ./run ide watch.

[radeusgd]

I added the default parameter to at (being 0 to choose first available column), but this has a downside that now we cannot nicely use table.at as a higher order function - if it gets no arguments it will use the defaults - so to pass it as an unapplied function we need to use the _ (or in the future ... or sth) to make sure defaults are not triggered.

I'm not saying this is a problem, especially as our GUI users will probably not use at in this mode too often. Knowing that our rule of thumb is to have default arguments wherever possible, I think we may keep it, but there is a trade-off to make.

Another thing I'm worried about is that if some users start learning about higher order functions they will can really easily get confused with how these default arguments work - but this a bigger problem of language design and I'm not sure if a better solution than we currently have is possible - it is always a trade-off. We probably just need to be careful about these defaults when writing user manuals, to make sure the users understand that they need the underscore to avoid defaults. We may also consider trying to improve the error messages somehow, as currently tracking the Not_Invokable_Error was a bit hard even for me, who I tihnk knows Enso pretty well.