Changelog for elm-street-0.2.0.0

Changelog

elm-street uses PVP Versioning. The changelog is available on GitHub.

Remove GHC 8.4.4 and 8.6.5 from CI / tested-with
Add GHC 8.10.7, 9.0.2 and 9.2.2 to CI / tested-with
Support Json.Decode.Value as primitive
Add primitive to represent NonEmpty lists as (a, List a) on elm side
Add overlapping instance for Elm String, to ensure that Haskell Strings are represented as String on Elm side (not as List Char)

#89: Regulate parenthesis on complicated types in encoder, decoder and type generation.

#80: Important: All encoders for constructors with fields now have tag due to aeson decoder on Haskell side.

Migration guide 1: Rename fields that will have tag name on the Elm side.

Migration guide 2: If you have manual ToJSON instances that communicate with Elm via generated decoders, you need to add tag field with the constructor name:
```
data User = User { ... }

instance ToJSON User where
    toJSON = [ "tag" .= ("User" :: Text), ... ]
```
#71: Breaking change: Remove overlapping instance for String.

Migration guide: Use Text instead of String.
#70: Use qualified imports of generated types and function in Elm generated files.
#74: Fix unit type typeRef encoder and decoder printers.
#72: Use consistent encoders and decoders for unary constructors.
#79: Implement cross-language golden tests.
#76: Support GHC-8.6.5. Use common stanzas.
#86: Refactor Elm.Print module and split into multiple smaller modules.
#73: Clarify the restriction with reserved words in documentation.
#90 Support converting 3-tuples.
#6: Test generated Elm code on CI.