Clean up infrastructure for merging partial objects

[hannobraun]

Add a MergeWith trait and implement it for all partial objects and various other types, making the merge infrastructure more regular and easier to use. I think leaning more heavily on merging might help with #1249, so this is good.

This also leaves the implementations of MergeWith for all partial object types quite formulaic, so it's possible to make those deriveable via a proc macro as a next step.