Lambda unpacking doesn't work for nested tuples

[bugarela]

Our tuple unpacking in lambdas only works for the top level.

module A {
  // works
  val foo = Set((1, true), ((2, false)).map((i, b) => if (b) i else 0)

  // doesn't parse
  val bar = Set(((1, "a"), true), ((2, "b"), false)).map(((i, s), b) => if (b) i else 0)
}