* Use cabal-doctest
- Haskell build tools run in slightly different environments (meaning different
package databases are available).
- The nixpkgs build for polysemy-plugin is failing due to a missing package
database, which causes the doctest to fail (more information here:
https://github.com/NixOS/nixpkgs/issues/71164).
- By using cabal-doctest we can expose the Haskell packages required to the
doctests no matter the build tool we're using.
* Use cabal-doctest in polysemy, build on GHC 8.8.1
- Use @googleson78 's changes to build polysemy on GHC 8.8.1, with slight
modifications. The source distribution is now found in "dist-newstyle/sdist",
so we've updated the command to point at that folder. Additionally, cabal
v2-install doesn't support installing .tar.gz files in the same way v1-install
did, so updated the command to use "cabal v1-install".
- Modified polysemy to use "cabal-doctest" and so overcome issues with the
doctest tests (see issue #258, PR #265).
