Deal with the MonadFail proposal

base-4.9 split MonadFail from Monad. Strengthen some type signatures to
require MonadFail instead of just Monad, derive MonadFail in relevant
places, and change a partial pattern match inside STM to one that
explicitly calls error. (STM is not a MonadFail; the user must
explicitly specify the desired semantics if a pattern match doesn’t work
out. In this case, the failing branch of the pattern should never be
reached, so crashing is fine.)

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