Make compiler config composable

[MattWindsor91]

When doing mutation experiments, I've found I want to maintain two configurations that have the same baseline config, and then different compiler definitions per machine. What would be nice is if we can have the baseline config in one file, the main config in another, and the mutant config in another, and have the latter two include the former.

I'm not convinced TOML is robust to this, so we might need to choose a different config file format.