[CDEC-500] Factor out example datatypes from Json+Bi tests

[mhuesch]

Description

As a first step for CDEC-500, factor out shared example datatypes, so that
I can add Address tests without duplicating code.

Commit message:

The `Bi` & `Json` modules had a number of datatypes which were shared
and could be factored out. This commit pulls them out into the
`ExampleHelpers` module.

I currently import the `ExampleHelpers` unqualified and without an
import list - this goes against the guidelines but seems ok to me in a
test module.

Linked issue

https://iohk.myjetbrains.com/youtrack/issue/CDEC-500

Type of change

• [~] 🐞 Bug fix (non-breaking change which fixes an issue)
• [~] 🛠 New feature (non-breaking change which adds functionality)
• [~] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• [~] 🔨 New or improved tests for existing code
• [~] ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• I have read the style guide document, and my code follows the code style of this project.
• If my code deals with exceptions, it follows the guidelines.
• I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.
• [~] CHANGELOG entry has been added and is linked to the correct PR on GitHub.
  ^ this is a minor PR; not putting it in the CHANGELOG.

Testing checklist

• [~] I have added tests to cover my changes.
• All new and existing tests passed.
  ^ refactoring, so existing tests should verify correctness.

[erikd]

LGTM!

[erikd]

💯

[Jimbo4350]

Good move, LGTM