Expose safe API from Cardano.Crypto.PackedBytes

[f-f]

Description

Due to the removal of the default DecCBOR implementation in Ledger, we need to expose PackedBytes and a few facilities to create/extract them. Unsafe functionality has been moved to the Internal module, which stays hidden.

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

[lehins]

Perfect! Thank you!