Use Foldable in MonadUtils and friends

This MR does three things

• generalizes the folds in MonadUtils to work over general Foldables, not just [].
• Replaces fold*Bag functions with their Foldable implementations (with specialisations for foldl and foldl' for Bag)
• Replaces foldls over Bags with foldl's.

This tidies some interfaces to make use of more general machinery that has been available for a while, and makes some headway on #16969 (closed) .

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