Change to override Prism behaviour for Option

[vlovgr]

The following example fails with a ClassCastException on v1.0.1.

import $ivy.`com.github.fd4s::vulcan:1.0.1`
import cats.implicits._
import vulcan.Codec

sealed trait FirstOrSecond

final case class First(value: Int) extends FirstOrSecond
object First {
  implicit val codec: Codec[First] =
    Codec[Int].imap(apply)(_.value)
}

final case class Second(value: String) extends FirstOrSecond
object Second {
  implicit val codec: Codec[Second] =
    Codec[String].imap(apply)(_.value)
}

val codec = 
  Codec.union[Option[FirstOrSecond]] { alt =>
    alt[None.type] |+| alt[Some[First]] |+| alt[Some[Second]]
  }
// codec: Codec[Option[FirstOrSecond]] = Codec([ "null", "int", "string" ])

codec.encode(Some(First(123)))

This is because the Prism instance for Option is derived using ClassTag, which (due to type erasure) considers Some[A] <: Option[S], regardless of A. This pull request solves the issue by creating explicit behaviour for Option. It does however not solve the issue more generally, and other parametric types still suffer from this issue.