Support for private constructors (via private Coercible instances)

[mrdziuban]

The Coercible instances currently generated by the @newtype macro are always publicly available in implicit scope. I think this makes sense when defining a newtype as a case class since a standard apply method will be generated, but when defining one as a class with a custom apply method, it allows external code to bypass any validation logic in that method. For example

// types.scala
package object types {
  @newtype class PositiveInt(i: Int)
  object PositiveInt {
    def apply(i: Int): Option[PositiveInt] = if (i > 0) Some(i.coerce[PositiveInt]) else None
  }
}

// test.scala -- this compiles but ideally it wouldn't
-1.coerce[types.PositiveInt]

Would it be possible to make the Coercible instances for a class only visible in implicit scope in the companion object? Or if that seems like too broad of a change, perhaps the logic could only apply when the class constructor is explicitly marked as private, e.g. @newtype class PositiveInt private (i: Int)?

Thank you for all your work on this library!