Add mempack and impspec

[lehins]

This PR adds two packages mempack and ImpSpec

Also it re-enables the test suite for scheduler, since the issue has been fixed and the
newly released version of scheduler-2.0.1.0 is now compatible with QuickCheck-2.15.x

I am not sure what the reason why random's test suite was disabled, but I am pretty sure it should be fine now.

CI failure seems totally unrelated to this PR

Checklist:

• Meaningful commit message, eg add my-cool-package (please don't mention build-constraints.yml)
• At least 30 minutes have passed since uploading to Hackage
• If applicable, required system libraries are added to 02-apt-get-install.sh or 03-custom-install.sh
• (recommended) Package have been verified to work with the latest nightly snapshot, e.g by running the verify-package script
• (optional) Package is compatible with the latest version of all dependencies (Run cabal update && cabal outdated)

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 --ignore-subdirs
  stack build --resolver nightly --haddock --test --bench --no-run-benchmarks

[DanBurton]

I am not sure what the reason why random's test suite was disabled, but I am pretty sure it should be fine now.

Reversed this in 3113722 , the reason was #7493 .

[lehins]

#7493 should have been resolved, since random-1.2.1.3 no longer depends on doctest:
https://github.com/haskell/random/blob/f8a47da565a7569029a83606df88eae9a7e04b9e/random.cabal#L129-L136

The doctests section is still there for backwards compatibility and it was completely removed in randaom-1.3.0

[lehins]

Anyways, I'll resubmit my PR without the commit that deletes random from the list of test suites executed by stackage.

Never mind, I only now realized that only one commit was reverted. All is good! 😄