WIP [ArrayManager] API: setitem to set new columns / loc+iloc to update inplace
xref indexing work item of #39146

This actually started as trying to get the tests passing for `tests/frame/indexing`, but while doing that I needed to make decisions on what the behaviour should be for setitem / loc / iloc assignment (so related other ongoing discussions about this). 

Very much a draft, but a good way to see what we actually want / the consequences of those choices.

cc @jbrockmendel 
This has some overlap with #39510; would that solve this issue? @jorisvandenbossche is #40380 helpful for this?  if not im going to mothball it Sorry for the slow reply here. I need to look into it again, but in any case, a part of the test changes of this PR was broken off and is already in master (#40323,  #40325). 

Another part of the code changes here were related to making loc and iloc update in place (https://github.com/pandas-dev/pandas/issues/39584)

I need to look again at #40380 to know if it would be helpful for this. 

(I generally plan to look at the indexing code starting next week, for the copy-on-write exploration)

 This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. this is quite old, happen to reopen if actively worked on.