API/CLN: setitem in Series now consistent with DataFrame
closes #4312

setitem in a frame that can change dtype
setitem_with_indexer now uses the Block.setitem machinery

side note - is there a difference between `_maybe*` methods and `_possibly*` methods?
 no...i think possibly -> maybe?
 @jtratner I know you are going to do a pep/cleanup.....so maybe add that to the list....
 @jreback not a big deal, just wondering if I was missing something :smile: 
