Source import of the vary package

[newhoggy]

Changelog

- description: |
    Source import of the `vary` package
# uncomment types applicable to the change:
  type:
  - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

This change is temporary until the vary package supports ghc-8.10 after the upstream PR is merged and/or ghc-8.10 support is no longer required.

How to trust this PR

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[github-advanced-security]

hlint found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

[carbolymer]

Nice. There are some HLint warnings and GHC errors though.

[carbolymer]

LGTM!

Small thing, let's mention in the context in the PR description that this change is temporary, and we should revert it and use vary package eventually after the upstream PR is merged and/or 8.10 support is no longer required

[newhoggy]

Addresses #566