Medium QoL changes

[carbolymer]

Changelog

- description: |
    - Deprecate `shelleyToAlonzoEraToShelleyToBabbageEra`, `alonzoEraOnwardsToMaryEraOnwards`, `babbageEraOnwardsToMaryEraOnwards`, `babbageEraOnwardsToAlonzoEraOnwards`.
    - Add `asType` for easier obtaining of `HasTypeProxy` proxies
    - Remove unneded `AsType a` functions' arguments
    - Remove some `ByronEra`-related dead code.

# 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 PR contains few QoL changes. It should be reviewed on commit-by-commit basis.

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

[Jimbo4350]

👍

[Jimbo4350]

This is my main contention: #825 (comment)

[carbolymer]

@Jimbo4350 I've pushed Remove AsType usage where not necessary commit, and added small changes to the others, so you might want to review everything again.

[Jimbo4350]

One comment, happy to approve after.