Add AvlTree

[Bodigrim]

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• (optional) Package is compatible with the latest version of all dependencies (Run cabal update && cabal outdated)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