Support ghc 8.10

[414owen]

Motivation for this change:

Well I ran nix-shell shell.nix over an hour ago and it still hasn't finished...

With this change, all you need to run to get a working environment is nix-shell -p ghc cabal llvmPackages_7.llvm pkg-config libffi, and your binary caches will work their magic.

I had to make some changes to llvm-hs, I'll try upstreaming them.

Some end-to-end tests are failing because of missing *.binary files. Not sure what the deal is. I'll see if the CI has more luck.