Does not build with ghc 8.6.3

Dependencies seem to need to be updated:
```
cabal new-build
Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: representable-functors-3.2.0.2 (user goal)
[__1] next goal: transformers (dependency of representable-functors)
[__1] rejecting: transformers-0.5.5.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0, transformers-0.4.3.0,
transformers-0.4.2.0, transformers-0.4.1.0 (conflict: representable-functors
=> transformers>=0.2 && <0.4)
[__1] trying: transformers-0.3.0.0
[__2] next goal: base (dependency of representable-functors)
[__2] rejecting: base-4.12.0.0/installed-4.1... (conflict: transformers
+/-applicativeinbase => base>=1.0 && <4.8)
[__2] rejecting: base-4.12.0.0, base-4.11.1.0, base-4.11.0.0, base-4.10.1.0,
base-4.10.0.0, base-4.9.1.0, base-4.9.0.0, base-4.8.2.0, base-4.8.1.0,
base-4.8.0.0, base-4.7.0.2, base-4.7.0.1, base-4.7.0.0, base-4.6.0.1,
base-4.6.0.0, base-4.5.1.0, base-4.5.0.0, base-4.4.1.0, base-4.4.0.0,
base-4.3.1.0, base-4.3.0.0, base-4.2.0.2, base-4.2.0.1, base-4.2.0.0,
base-4.1.0.0, base-4.0.0.0, base-3.0.3.2, base-3.0.3.1 (constraint from
non-upgradeable package requires installed instance)
[__2] fail (backjumping, conflict set: base, representable-functors,
transformers)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: representable-functors, base,
transformers
```