Add CanEqual typeclass instance for Seq to match Nil case

[kevin-lee]

Add CanEqual instance for Seq to match Nil case.

This PR solves the similar issue as what #12419 solves, but it's for pattern matching on List.
With -language:strictEquality compiler option or import scala.language.strictEquality, the following case doesn't compile

List(1, 2, 3, 4, 5) match {
  case n :: ns =>
    println(s"head: $n, tail: ${ns.mkString("[", ",", "]")}")
  case Nil =>
    println("empty")
}

with this compile-time error

    case Nil =>
         ^^^
Values of types object scala.collection.immutable.Nil and List[Int] cannot be compared with == or !=.
I found:

    CanEqual.canEqualSeq[Nothing, Int](/* missing */summon[CanEqual[Nothing, Int]])

But no implicit values were found that match type CanEqual[Nothing, Int].

This PR solves it.

[dwijnand]

LGTM. Would be nice to tweak the comments, but this shouldn't slip to 3.2.0 on that basis!

[kevin-lee]

@dwijnand I've rebased the upstream mater and updated the commit as suggested. Thank you!

[smarter]

I don't understand why the original given isn't enough to match Nil /cc @odersky

[dwijnand]

It's the same logic as the Option/Either PR, you get:

    case Nil =>
         ^^^
Values of types object scala.collection.immutable.Nil and List[Int] cannot be compared with == or !=.
I found:

    CanEqual.canEqualSeq[Nothing, Int](/* missing */summon[CanEqual[Nothing, Int]])

But no implicit values were found that match type CanEqual[Nothing, Int].

[nicolasstucki]

@kevin-lee could you rebase this PR on master? We aim merge it for 3.1

[kevin-lee]

@nicolasstucki Sure, I will do it soon.

[kevin-lee]

@nicolasstucki Done!

[kevin-lee]

and done again.

[odersky]

It would be good to add a comment why the new method is needed to the code. It is explained in the comments to the PR.