make eachcol default to false

[bkamins]

This cleans up deprecations and respects JuliaLang/julia#29749.

It would not hurt if someone checked if I had put right Julia version in the condition. Thanks!

[bkamins]

This should be good for a review. I missed deprecation in mapcols in the #1614; I fix it here - I would leave it as is (the only problem is that mapcols prints a depwarn).

When we merge this then the short-term plan is: #1603 and after it fixing #1608.

[bkamins]

@nalimilan
The PR is in red, because coverage dropped (probably because I have removed some code and some unnecessary tests). The tests go through in green.

[nalimilan]

This looks good to me, but I'm not sure we should merge it right now since it will break code that hasn't been ported yet, and there's no hurry to change this: old code that used eachcol(df) will have to do eachcol(df, true) anyway. AFAICT the only advantage of doing this (apart from cleanup) is to allow people to write eachcol(df) instead of eachcol(df, false). Better make a release removing deprecations which people cannot turn off (in indexing code).

[bkamins]

OK - let us put it on hold. I will keep it open to remember what has to be changed. How releases would you wait with fixing this?

[nalimilan]

Not sure, but I would at least make a release removing the indexing deprecations first, so that people can upgrade without too many changes to silence them.

[bkamins]

Do we want to finish the deprecation for eachcol(::DataFrame) or we wait one more release?

[bkamins]

@nalimilan - I think that enough time has passed for the deprecation. If you are OK to merge it now I will review this PR to make sure and make it ready for merging.

[nalimilan]

Fine with me.