Another way of improvement to the 101_calculator, besides the "Reverse Polish Notation" one declined by DavePearce

1. This piece of code will correct the order problem, that it should be 5 - 1 - 1 = (5 - 1) - 1, but actually 5 - 1 - 1 = 5 - (1 - 1) in the orginal Main.whiley.

2. A proper way is modification to the parsing stage, but that will cause a infinite loop in the compiler, which is compiler's own bug (https://github.com/DavePearce/Whiley/issues/250).

3. My sulotion is playing a trick in the evluation stage.