I2S setup config update

[joshua-laughner]

I want to make two changes to the input files the various "i2s-prep" programs use:

1. Transition from JSON to TOML: other parts of EGI-RS use TOML, so I'd prefer to be consistent. TOML is easier to read IMO.
2. Make the coordinate enum internally tagged: Right now, this is untagged. Both for consistency and future-proofing, it should have a "type" field that determines which enum variant to deserialize into.