Add more pos and neg tests

Tests suggested by @retronym's comments on issue #897.

Author: odersky

tests/neg/function-arity.scala

@@ -18,5 +18,7 @@ object Test {
 
   unary[(Int, Int)]((x: String, y) => ()) // error
 
-
+  def foo(a: Tuple2[Int, Int] => String): String = ""
+  def foo(a: Any => String) = ()
+  foo((a: Int, b: String) => a + b) // error: none of the overloaded alternatives of method foo match arguments (Int, Int)
 }

tests/pos/function-arity.scala

@@ -14,5 +14,7 @@ object Test {
   unary(1, 2, (x: Int, y: Float) => x)
 
   val xs = List(1, 2, 3)
+  def f(x: Int, y: Int) = x * y
   xs.zipWithIndex.map(_ + _)
+  xs.zipWithIndex.map(f)
 }