Minimal support for dependent case classes

[smarter]

This lets us write:

    trait A:
      type B

    case class CC(a: A, b: a.B)

Pattern matching works but isn't dependent yet:

    x match
      case CC(a, b) =>
        val a1: A = a
        // Dependent pattern matching is not currently supported
        // val b1: a1.B = b
        val b1 = b // Type is CC#a.B

(for my usecase this isn't a problem, I'm working on a type constraint API which lets me write things like case class CC(a: Int, b: Int GreaterThan[a.type]))

Because case class pattern matching relies on the product selectors _N, making it dependent is a bit tricky, currently we generate:

    case class CC(a: A, b: a.B):
      def _1: A = a
      def _2: a.B = b

So the type of _2 is not obviously related to the type of _1, we probably need to change what we generate into:

    case class CC(a: A, b: a.B):
      @uncheckedStable def _1: a.type = a
      def _2: _1.B = b

But this can be done in a separate PR.

Fixes #8073.

[noti0na1]

I am doing some experiment with dependent pattern match in CC, and this PR is useful for me as well!

I modified the desuger so _1, _2, etc will have better types: always use singleton types when the field is not var. I haven't finished because I also need to modify the typer so the selections in the cases are binded to something.

[smarter]

@noti0na1 Awesome! Let me know how it goes.