Index aware PlutusPurpose

[lehins]

Description

• Add AsIxItem, which allows us having a PlutusPurpose be both item and index aware, thus avoiding the reverse lookup of an item in order to find its index.
• Fix ConwayTxBody pattern synonym, by changing its certificates arguments to OSet
  from a StrictSeq
• Add SpendingPurpose, MintingPurpose, CertifyingPurpose, RewardingPurpose,
  VotingPurpose and ProposingPurpose pattern synonyms to allow era agnostic handling of PlutusPurpose

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• 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)
• 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)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

Looks good to me 🤯