macroFromTo should cope with type-parameter fields

[jducoeur]

There are lots of use cases here, but abstractly the goal is to be able to say this:

case class MyWrapper[T : FromTo](
  body: T
)

object WithKafkaHeaders {
  implicit def wrapperFromTo[T : FromTo]: FromTo[MyWrapper[T]] = macroFromTo
}

Currently this fails at compile time, with a diverging implicit expansion error. It isn't clear what the macro is trying to do right now, but it clearly isn't quite what I'd intuitively expect. Hopefully we can get this working right.