Clean up table operations code

Inspired by #5811, and after #5841, there is a lot of opportunity to clean up the code. This includes getting rid of the separate _vstack, _hstack, _join methods and so on. But in order to keep the diffs in #5841 reviewable defer this.

Also there are three versions of common_dtype floating around, and possibly opportunity for cleanup with regards to exception handling there (like places where the exception is re-raised).