Eliminate compilation warnings

[chameco]

It might also be nice to reintroduce -Werror in saw-script.cabal, although I'm not sure if that would negatively impact others' development workflows.

[brianhuffman]

Instead of putting -Werror in the cabal file (which can cause headaches for developers, as you say) maybe we should be doing our CI builds with -Werror on the command line. This way we can allow warnings in local builds by default, but we can disallow merges to master that introduce warnings.

[chameco]

This is unfortunately complicated a bit by haskell/cabal#3883. Other projects backlinking to that issue (e.g. github/semantic#428) appear to circumvent the issue by adding a CI-specific cabal.project that adds -Werror to ghc-options.

Considering this, I suspect that appending

package saw-script
  ghc-options: -Werror

to cabal.project during Travis setup is a reasonable (but ugly ☹️) solution.