report the top-level goal when hitting the cutoff (fixes #65)
report the top-level goal when hitting the cutoff (fixes #65)

Formerly cutoffExceeded() reported the current goal, which was not
necessarily part of the problematic cycle.  For example, given

    class C a where
        f :: a -> a

    instance (Eq a, C a) => C a

    test = f True

it reported that it could not prove Eq Bool, even though this is
provable in one step.

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