re-enable `ghc-typelits-*` type-checker plugins
re-enable `ghc-typelits-*` type-checker plugins

Now that `ghc-tcplugins-extra-0.2.4` is in nightly:

- `ghc-typelits-natnormalise-0.5.9`, and
- `ghc-typelits-knownnat-0.4.1`

Can be build on GHC 8.4.

- `ghc-typelits-extra-0.2.4` can now also build on GHC 8.4 now that its (transitive) dependencies can build on GHC 8.4
fimad/stackage,Shimuuar/stackage,mstksg/stackage,iand675/stackage,peti/stackage,fpco/stackage,wereHamster/stackage,stackbuilders/stackage,jonschoning/stackage,creichert/stackage,lwm/stackage,simonmichael/stackage,garetxe/stackage,athanclark/stackage,mgajda/stackage,massysett/stackage,mrkkrp/stackage,Daniel-Diaz/stackage,asr/stackage,michalkonecny/stackage,int-index/stackage,clinty/stackage,ethercrow/stackage,spencerjanssen/stackage,soenkehahn/stackage