Replace hocon configuration library

[Bathtor]

It seems the person maintaining the hocon library we've been using for configuration has stopped maintaining it. And it has an upcoming Rust incompatibility in its parser dependencies, that'll bite us in the arse eventually. So we need to either replace it, or take over the maintenance and fix it.

TODO

• [x] Look for alternative hocon libraries and list them
• [x] Look at alternative config formats
• [ ] Make a decision

[Bathtor]

Looked for more libraries, but it looks like this is the only one on crates.io. The other HOCON thingy there is a fork of the same library, but even older.

That leaves considering new options:

• YAML may fill the role and has good support in the Rust ecosystem
• JSON is always an option

[Bathtor]

TOML is probably the best option, given it's widespread use in the Rust ecosystem.