Draft: Fix build with JS backend

[hsyl20]

Fix the build with the JS backend.

Just here to avoid duplicating work.

Known issues:

• don't fix any FFI import (GHCJS inline syntax isn't supported by the JS backend)
• don't maintain backward compatibility
• not tested

[alaendle]

Many thanks for this draft, just out of curiosity I tried to make the test-suite pass (https://github.com/alaendle/ghcjs-base/tree/renovate) - however I have the feeling that this inevitable reveals all the current shortcomings (e.g. incomplete JavaScript support in text - https://github.com/haskell/text/pull/536 - on this one I have stopped); and also the easy to mitigate https://github.com/haskellari/splitmix/pull/75. I think this comment is not a news for people directly involved, but maybe it's helpful to outsiders.

[hamishmack]

Replaced by #135