Asking for code review - contract programming

[chopchop]

Hello

So I modified the code according to Ali's review. For those 
interested in contracts, it took me a while to find the 
documentation about the new "expression-based" syntax that Ali is 
talking about:

https://dlang.org/changelog/2.081.0.html#expression-based_contract_syntax


Also, the game "kind of" work on Linux, you must:
- download the project file
- cd to SnakeD/
- ```shell
   dub run
   ```
- IMMEDIATELY hit the UP arrow key
- then you can move with the UP DOWN LEFT RIGHT arrow key, but 
there is a bug which is unfortunately outside the scope of my 
project. It seems that on Linux the terminal (arsd lib) is only 
updating when pressing a key. Adam is you read this...
- You can also go to ground.d and update the values of 
playgroundWidth/Height to have a bigger playground, as a 
work-around.

I will let another week pass, if everybody wants to make a review 
/ give some feedback, then I will work on comparing the DO 
requirements to what I obtained with d contract practices.

Then maybe a few updates to the code

Then conclusion

CHeers