Add Show (AsType a) instance

[carbolymer]

Changelog

- description: |
    Add `Show (AsType a)` instance
# 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...
# uncomment at least one main project this PR is associated with
  projects:
   - cardano-api
  # - cardano-api-gen
  # - cardano-rpc
  # - cardano-wasm

Context

Not really used anywhere yet. It was an useful instance when debugging transaction deserialisation for IntersectMBO/cardano-cli#1304

-- >>> show AsByteString
-- "AsType ByteString"

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