Update the "disjointX" variants to use IntMap.disjoint

IntMap (and Map) have gained dedicated disjoint functions in containers-0.6.2.1, which will be bundled with GHC-8.8.

That means that we can start using these functions in GHC-8.12.

At that point we should update disjointUFM, disjointUDFM etc. to use the new functions.

Edited Jun 27, 2019 by Simon Jakobi

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