By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Use stm-containers for plugin and mutation-mode shared state Replace the IORef-of-Map process globals in the mutation plugin with stm-containers maps, and collapse the config cache to a single TVar slot now that every module in one GHC invocation sees the same --config flag. Also swap the mutation runner's per-group results TVar for an StmContainers.Map keyed by group index.