ToJSON instance for TxValidationError

[newhoggy]

Changelog

- description: |
    `ToJSON` instance for `TxValidationError`
# 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
  # - improvement    # QoL changes e.g. refactoring
  # - 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 is needed for reporting errors in machine readable JSON.

Note that the error tree is not completely converted to JSON structure. Some sub-branches remain as strings because the ledger does not export enough symbols to go all the way down to the leaves.

In any case, these instances are a stop-gap until the instances are implemented in the ledger proper.

How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

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

mistake

[smelc]

I think a little motivation comment would be helpful in the long term. I believe here the main point of ShowOf is that you want to capture when Show can be lifted to ToJSON, right? I think stating this motivation would help maintenance in the long term (and onboarding of newcomers to the codebase (speaking for a friend)).

[smelc]

LGTM