Should merge always return a Dict?

[garborg]

Right now it does, e.g. when merging two ObjectIdDicts. I'd have expected something like calling similar on the first container.

Users can always use merge! if they care to pick a container, so I don't have a strong opinion -- just raising it before 1.0 in case anyone does.

[nalimilan]

We could reuse the same promotion mechanism as for cat (#20815).