Reenable servant

[maksbotan]

Checklist:

• Meaningful commit message, eg add my-cool-package (please not mention build-constraints.yml)

• At least 30 minutes have passed since uploading to Hackage

• On your own machine, you have successfully run the following command (replace $package with the name of the package that is submitted, and $version with the version of the package you want to get into Stackage):

  ./verify-package $package # or $package-$version
  

The script runs virtually the following commands in a clean directory:

  stack unpack $package-$version  # $version is optional
  cd $package-$version
  rm -f stack.yaml && stack init --resolver nightly
  stack build --resolver nightly --haddock --test --bench --no-run-benchmarks

Note that some of these packages have Hackage revisions that fix version bounds.

I'm going to update the rest of Servant family in the next few days.

Fixes haskell-servant/servant#1340