chore: modify MonadStats to not extend parents multiple times
#169
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MonadStats was extending `MonadLiftT` twice, but the two parents were defeq. This was a hack to get around the fact that the parent instances were providing `MonadLiftT` instances rather than a `MonadLift` instance, breaking `MonadLiftT` inference. This changes it to *not* extend `MonadLiftT`, instead embedding it as a field, and then writing an explicit `MonadLift` instance. This is motivated by [lean4#5770](leanprover/lean4#5770), which will log warnings when structures extend parents multiple times.
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MonadStats was extending
MonadLiftTtwice, but the two parents were defeq. This was a hack to get around the fact that the parent instances were providingMonadLiftTinstances rather than aMonadLiftinstance, breakingMonadLiftTinference.This changes it to not extend
MonadLiftT, instead embedding it as a field, and then writing an explicitMonadLiftinstance. While it could still extend theMonadLiftTinstance, it is better this way because no one should be providingMonadLiftTinstances.This is motivated by lean4#5770, which will log warnings when structures extend parents multiple times.