Use heterogeneous lists in code generated by Witty derive macros

[jvff]

Motivation

The code generated by Witty derive macros is non-trivial and is mostly equivalent to the implementation for frunk's heterogeneous lists.

Proposal

Refactor the Witty derive macros to generate code that use heterogeneous lists instead of code to manually handle each field in a type. This reduces the amount of code generated, centralizes the more complex code in the implementations for frunk types and hopefully improves readability of the generated code.

Test Plan

The unit and integration tests that already exist cover the behavior which the refactoring must not change.

Links

This is an unexpected extra work related to of #906.

Release Plan

• All good!
• Need to bump the major/minor version number in the next release of the crates.
• Need to update the developer manual.
• This PR is adding or removing Cargo features.
• Release is blocked and/or tracked by other issues (see links above)

Reviewer Checklist

• The title and the summary of the PR are short and descriptive.
• The proposed solution achieves the goals stated in the PR.
• The test plan is reproducible and provides sufficient coverage.
• The release plan is adequate.
• The commits correspond to distinct logical changes.
• The code follows the coding guidelines.
• The proposed changes look correct.
• The CI is passing.
• All of the above!

[ma2bd]

Makes sense!

[ma2bd]

The unit tests help a lot!