Add ghcjs support

[hamishmack]

#3316 rebased and squashed

[michaelpj]

Haven't looked in detail, just wondering why we need vendored copies of things instead of using source-repository-packages?

[angerman]

Haven't looked in detail, just wondering why we need vendored copies of things instead of using source-repository-packages?

contrib contains modified copies. Having them in separate repos would cause quite some churn until we know exactly what we need fixed (and how). They will eventually have to go.

[michaelpj]

This is going to need some changes for the new repository split. I think most of the hard stuff is for plutus-apps, but it would still be good to build the stuff here with GHCJS so we don't accidentally break it. I hope that's a clean-ish subset of what's in this branch?