Optimize op-assignment statement.

scripts/loop.rhai

@@ -6,7 +6,7 @@ let x = 10;
 loop {
     print(x);
 
-    x = x - 1;
+    x -= 1;
 
 	if x <= 0 { break; }
 }

scripts/speed_test.rhai

@@ -7,7 +7,7 @@ let x = 1_000_000;
 print("Ready... Go!");
 
 while x > 0 {
-    x = x - 1;
+    x -= 1;
 }
 
 print("Finished. Run time = " + now.elapsed() + " seconds.");

scripts/while.rhai

@@ -4,5 +4,5 @@ let x = 10;
 
 while x > 0 {
     print(x);
-    x = x - 1;
+    x -= 1;
 }