Build failure with ghc 8.4

[DanBurton]

Add an instance for semigroup.

Building library for heap-1.0.3..
[1 of 3] Compiling Data.Heap.Internal ( Data/Heap/Internal.hs, dist/build/Data/Heap/Internal.o )

Data/Heap/Internal.hs:69:10: error:
    • Could not deduce (Semigroup (HeapT prio val))
        arising from the superclasses of an instance declaration
      from the context: Ord prio
        bound by the instance declaration at Data/Heap/Internal.hs:69:10-46
    • In the instance declaration for ‘Monoid (HeapT prio val)’
   |
69 | instance (Ord prio) => Monoid (HeapT prio val) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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