Record-wrapping newtypes should produce MoatStruct

[tadfisher]

Example:

newtype Newtype = Newtype {newtypeField :: Text}

Aeson serializes this as a JSON object like:

{"newtypeField":"foo"}

But Moat generates Kotlin code that deserializes this as its wrapped value, so we expect "foo" on the wire. The Mercury codebase has a ton of newtypeToStruct calls to coerce these into MoatStruct, and backend engs don't really know about this nuance so it's forgotten.

This changes mkNewtype to produce a MoatStruct if the wrapped constructor is a record constructor. In addition, this lifts the restriction that newtypes can't wrap records with multiple fields.

I am not sure if this breaks Swift codegen. If I'm correct about Aeson's output, then generating untagged structs shouldn't make a difference.