Use ghc-proofs package in testsuite

[kosmikus]

This is connection with the work on static-case and optimisation (see #40).

By now, I think it's feasible to get the static-case branch into a shape where it produces mostly the same code as handwritten code. Once we have that, we should be able to "prove" this using the ghc-proofs package and thereby add regression tests for these properties.

/cc @nomeata @andorp

[nomeata]

See https://github.com/kcsongor/generic-lens/commit/3fddfc5b9fde0904e452a1866515fd493b914c67 for an example where ghc-proofs is used.