3f249f18

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.

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