mutable decl syntax change

examples/programs/index_int_constraint_error/main.bro

@@ -1,6 +1,6 @@
 main func() i32 {
-	items [3]mut i32 = undefined
-	i int = 1
+	items [3]mut i32 := undefined
+	i int := 1
 	items[i] = 42
 	return 0
 }