Simple enhancement for pattern matching with capturing types

[noti0na1]

The bind variable should carry the capture set from the selector during pattern matching.

import language.experimental.captureChecking

trait A

case class B(x: AnyRef^) extends A

def test =
  val x: AnyRef^ = new AnyRef
  val a: A^{x} = B(x)

  val y1: A = a match
    case b: B => b // error: (b: B) becomes B^{x} implicitly

  val y2: A^{x} = a match
    case b: B => b // ok

  val x3: AnyRef = a match
    case B(x2: AnyRef) => x2 // error: we lose some information about field x, but it still cannot be pure

  val x4: AnyRef = a match
    case b: B => b.x // error