According to stackage.org, 13.11 should be the last LTS release with GHC 8.6.3,
but strangely that also uses 8.4.4. The now selected 13.10 should really depend
the older GHC.

The main reason being that we need a newer criterion version to pull
in fixes for bugs that affected our benchmarks and the easiest way to
get that was to bump the stack resolver which incidentally also
upgrades the compiler.

The performance implications of this change are:

  - Microbenchmarks stay more or less the same, parsing benchmarks
    seem to get a little bit slower.
  - Real-World benchmarks on PRISM examples seem to get a bit faster
    overall.

This adds options, help text, etc to the `prism-converter` binary.

This already has https://github.com/mrkkrp/hspec-megaparsec/pull/6
which makes running tests in emacs much more useful.