lang: funcs: structs: Map indexes use half the integers

We want the pattern to be key:0, val:0, key:1, val:1, and so on... This
was previously using 0,1,2,3...

When we use Call directly, we need to fix this. Previously this was dead
code which is why the bug wasn't caught.

lang/funcs/structs/composite.go

@@ -292,13 +292,16 @@ func (obj *CompositeFunc) Call(ctx context.Context, args []types.Value) (types.V
 		}
 
 	case types.KindMap:
+		count := 0
 		for i, arg := range args {
 			if i%2 == 0 {
-				key1 := fmt.Sprintf("key:%d", i)
+				key1 := fmt.Sprintf("key:%d", count)
 				st.V[key1] = arg
 			} else {
-				key2 := fmt.Sprintf("val:%d", i)
+				key2 := fmt.Sprintf("val:%d", count)
 				st.V[key2] = arg
+
+				count++ // increment for next even number
 			}
 		}