tuple-zipped.scala: support for case-syntax of lambda arguments

[DarkDimius]

[info] /Users/dark/workspace/dotty/tests/partest-generated/run/tuple-zipped.scala:27: error: wrong number of parameters, expected: 2
[info]       val sum1 = (cc1, cc2).zipped map { case (x, y) => x + y } sum
[info]                                        ^
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/tuple-zipped.scala:27: error: type mismatch:
[info]  found   : Nothing(x)
[info]  required: ?{ + : ? }
[info] Note that implicit conversions cannot be applied because they are ambiguous;
[info]  both method Character2char in object Predef$ and method Byte2byte in object Predef$ convert from Nothing(x) to ?{ + : FunProto(<y:Nothing>):Nothing' }
[info]       val sum1 = (cc1, cc2).zipped map { case (x, y) => x + y } sum
[info]                                                         ^
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/tuple-zipped.scala:27: error: ambiguous implicits: both object FloatIsFractional in object Numeric$ and object ByteIsIntegral in object Numeric$ match type scala.math.Numeric[Nothing'] of parameter num of method sum in trait TraversableOnce
[info]       val sum1 = (cc1, cc2).zipped map { case (x, y) => x + y } sum
[info]                                                                    ^
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/tuple-zipped.scala:34: error: wrong number of parameters, expected: 3
[info]       val sum1 = (cc1, cc2, cc3).zipped map { case (x, y, z) => x + y + z } sum
[info]                                             ^
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/tuple-zipped.scala:34: error: type mismatch:
[info]  found   : Nothing(x)
[info]  required: ?{ + : ? }
[info] Note that implicit conversions cannot be applied because they are ambiguous;
[info]  both method Character2char in object Predef$ and method Byte2byte in object Predef$ convert from Nothing(x) to ?{ + : FunProto(<y:Nothing>):?{ + : FunProto(<z:Nothing>):Nothing' } }
[info]       val sum1 = (cc1, cc2, cc3).zipped map { case (x, y, z) => x + y + z } sum
[info]                                                                 ^
[info] /Users/dark/workspace/dotty/tests/partest-generated/run/tuple-zipped.scala:34: error: ambiguous implicits: both object FloatIsFractional in object Numeric$ and object ByteIsIntegral in object Numeric$ match type scala.math.Numeric[Nothing'] of parameter num of method sum in trait TraversableOnce
[info]       val sum1 = (cc1, cc2, cc3).zipped map { case (x, y, z) => x + y + z } sum
[info]                                                                                ^
[info] three warnings found

[smarter]

Fixed by #887, leaving this open until we move this test out of pending.