[WIP] make more integer types DefaultsSerializable

[DivineDominion]

I started with UInt, but the obvious next step would be to add Int64/32/16/8 and UInt64/32/16/8.

The code is pretty repetitive. In other projects, we used a code generator for this to prevent typos and code that's getting on one's nerves to maintain. But for 8 new types and their tests, we could stick with manual implementations just as well.

• Do you want to support the other types out of the box at all, anyway?
• Do you want manual or generated code?