Make the current `base` buildable with GHC 10.0

Wolfgang Jeltschrequested to merge
wip/jeltsch/base-buildable-with-ghc-10-0 into master

In the medium run, we’d like to provide base versions that work with different versions of GHC each. As a first step to achieve this, we should make sure that the current development version of base is buildable with the latest non-development version of GHC, which is currently GHC 10.0, to be released soon. This contribution establishes compatibility of base with GHC 10.0.

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