Create instances for ToContent Void, ToTypedContent Void

[lykahb]

Some handlers never respond with the success response. They always redirect, sendFile or return some other error. Giving them the type Handler () implies that they may return an empty success response. The type Handler Void is more precise and helps to understand the handler behavior at a glance.

---

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[snoyberg]

LGTM! Just out of curiosity, what’s the use case where this came up?

[lykahb]

There were two use cases where the specific type was used only to make the handler content type monomorphic:

• a handler with sendResponse ("application/pdf" :: ContentType, content). I replaced this exception-based response with Handler (ContentType, Content).
• a handler that sends sendStatusJSON Status.status202 payload on its happy path. For that one the Handler Void is the most fitting type

[snoyberg]

Got it thanks!