Enable deriving Reject for enums with fields

[Bargsteen]

Purpose

Addresses a primary part of #78 by allowing the user to derive Reject for enums with fields (if the fields implement Serial).
We will likely also need to update some tools and documentation in order for this task to be finished entirely.

Changes

• Add the ability to derive Reject for enums with fields (if the enum implements Serial).

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[limemloh]

LGTM

Only a suggestion for an improvement