This ensures that all of the `dhall-*` libraries build against
GHC 8.6.1, but still generates tarballs using GHC 8.4.3 and
uses GHC 8.4.3 for development
... as requested by @jneira so that Dhall will continue to work as a
dependency of Eta, which is built using GHC 7.10.3
This adds CI support for testing the build against GHC 7.10.3 and also
fixes issues building against that GHC version that were caught in the process