Replace serializer

**What do you want to see?**
Replace the old, slow serializer with something more performant - say, the built-in .NET Core JSON serializer which is based on Newtonsoft's serializer.

**How would it help players?**
Game saves and loads faster

**Alternatives**
Other serializers?

**Screenshots and mockups**
N/A

**Things to consider**
Need to make sure all game file types can be saved and loaded, but the big one is savegames. I've had issues with Newtonsoft in the past which is why I created the custom serializer to begin with; I need to make sure anything we replace it with is reliable.