Support various encoding styles of Aeson

[chiroptical]

Introduced in #35, aeson allows for different encoding styles. We should support then for the languages where it makes sense. The styles are documented here

[tadfisher]

A good one for the Kotlin backend is ObjectWithSingleTag, because newtypes are collapsed by Kotlin Serialization. For example

value class Foo(val field: String)

data class Bar(val foo: Foo)

Json.encodeToString(Bar(Foo("hello")))
// -> { "foo": "hello" }

data class Foo(val field: String)

data class Bar(val foo: Foo)

Json.encodeToString(Bar(Foo("world")))
// -> { "foo": { "field": "hello" } }

This currently forces a workaround to either manually implement ToMoatData using MoatStruct, or to write a function like

-- | Given a newtype, generate a struct instead.
newtypeToStruct :: MoatData -> MoatData
newtypeToStruct MoatNewtype {..} =
  MoatStruct
    { structName = newtypeName
    , structTyVars = newtypeTyVars
    , structInterfaces = newtypeInterfaces
    , structProtocols = newtypeProtocols
    , structAnnotations = newtypeAnnotations
    , structFields = [newtypeField]
    , structPrivateTypes = []
    , structTags = []
    }
newtypeToStruct _ = error "newtypeToStruct: only works for newtypes"