Add optional custom ctor via voidable sum pattern

[JordanMartinez]

Description of the change

Fixes #85, except not quite... Void doesn't have an Eq instance, so neither does JsonDecodeError'. Thus, the test won't compile. Moreover, I'm not sure Void should have an Eq instance. While we could update the test to not use ==, will this have implications for the rest of the ecosystem that'd we want?

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[jy14898]

@JordanMartinez Void does have an Eq instance. I think it's fine to have that instance. Firstly, const true passes the Eq laws for any type, but more importantly you can never observe law breaking if your class has a member that must receive a value of Void