cardano-base: initialize with CardanoFeatureFlag

[amesgen]

Description

This PR adds a new library cardano-base that, for now, only contains a very simple module with a feature flag sum type.

In the future, more functionality might be added/moved to cardano-base (potentially as a sublibrary), in contrast to the existing narrowly-scoped packages in this repo.

Addresses tweag/cardano-peras#96

See eg IntersectMBO/ouroboros-network@c330acc for a potential real-world usage example

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Self-reviewed the diff

[nfrisby]

This looks great to me; addressed all of the concerns I voiced during the call in which this idea was proposed.

Maybe add Read to the deriving clause, for easier QuickCheck debugging cycle.

[lehins]

Looks good. Just a few minor changes requested

[lehins]

Thank you!