yesod-form: Add Monad AForm instance for transformers >=0.6

[TeofilC]

This is required in order to have a MonadTrans instance.

Fixes #1794

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

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• 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)

[TeofilC]

I think this won't be tested on CI atm. I'm not sure what the best way forward is on that. Maybe adding a snapshot to the repo that adds new mtl and transformers to nightly

[snoyberg]

LGTM, thanks!