This gives RAR the ability to save off a modified state file and load it for later use.

Still to do:
1) Verify that the precomputed cache read in is real.
2) Change logging such that it will log messages instead of errors or warnings if there are problems with deserializing the precomputed cache
3) Comments at relevant points
4) Switch serialization mode to another form (json?)
5) Validation + performance tests
6) Have SDK opt in

It was previously serializable via BinaryFormatter. This makes it serializable via System.Text.Json's JsonConverter.

Add support for serializing SystemState using System.Text.Json rather than the older BinaryFormatter.

… just when serializing and deserializing the precomputed cache

I have not yet measured how long it takes to read a MVID vs. reading and parsing its references, nor have I made unit tests.

…ache

...although they don't work yet

…son-serialization

…son-serialization

The version of Arcade MSBuild currently uses uses Roslyn version 3.3.1. This updates that, but it should be removed when we've updated to a more modern version of arcade.

…into json-serialization

…son-serialization

… into serialization-only