Is there any build queue, or some way to switch production without wasting hammers?

As far as I can tell, there is, sadly, not a build queue available in Civ VI.

However, I can confirm that switching production does not reset your progress. I worked on a Builder for a few turns, then switched. After finishing the latter target and going back to choose production, I had the same production applied to a Builder, and was able to complete it from where I had left off.

Something to note: When building a District of any kind, the destination tile for that District is set whenever you confirm its placement. Changing production targets, then switching back to the District will not allow you to change the destination tile for that District.

Source: Anecdotal gameplay experience

There is no show queue in Civ, which makes me sad because I abused that feature in Civ V.

However, just like Civ 5, you won't lose the production you put into things, and Civ 6 actually SHOWS this a bit better by showing a progress bar on things you'd previously started.