JET-34 Require a return statement in a function with a block body

JET-77 Require a return type annotation for non-Unit returning functions with block bodies

idea/testData/codegen/classes/inheritance.jet

@@ -29,5 +29,5 @@ fun box() : String {
     val p4 = P4(240, y)
     if (p4.x + p4.y != 239) return "FAIL #7"
 
-    "OK"
+    return "OK"
 }

idea/testData/codegen/controlStructures/for.jet

@@ -5,5 +5,5 @@ fun concat(l: List<String>): String? {
   for(s in l) {
     sb.append(s)
   }
-  sb.toString()
+  return sb.toString()
 }

idea/testData/codegen/controlStructures/forInArray.jet

@@ -3,5 +3,5 @@ fun concat(l: Array<String>): String? {
   for(s in l) {
     sb.append(s)
   }
-  sb.toString()
+  return sb.toString()
 }

idea/testData/codegen/localProperty.jet

@@ -2,5 +2,5 @@ fun f(a:Int) : Int {
    val x = 42
    val y = 50
 
-   y
+   return y
 }