Add voidEncoder/HasInitialObject

[alexfmpe]

I have:

• Based work on latest develop branch
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• Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• Run the test suite: $(nix-build -A selftest --no-out-link)
• Updated the changelog
• (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[maralorn]

Does this have any use?

[ymeister]

What's the idea behind this HasInitialObject aside from being a voidEncoder helper?

[alexfmpe]

Does this have any use?

Yes.

Given void1Encoder can be decomposed into it, the implementation has at least as much use as void1Encoder. More generally, this is an identity not that much different from mempty.
Void can be used as an neutral element of Either or absorbing element of (,). I've used Void as the obelisk backend route for a frontend-only webapp.

That said, I added the instance itself mostly out of completeness