Implement smooth movement.

[ceranco]

This PR adds smooth movement to the game as per #3.

[ceranco]

So funny story,
I thought this would be a breeze to implement, but after making an initial effort it turns out that making a retro fixed-axis game like snake move 'smoothly' makes it feel really janky and is pretty non-trivial when using a simple sprite-sheet like I am 😞

I think that way to fix all the jankiness is to wait until there is animation. This will enable making gradual turns and emphasizing where on the grid the snake is even if the movement isn't tied to the update interval.