Added growth and speed change behaviors.

This update brings new behaviors to the growth and speed of the snake.

Snake Growth

Previously, the snake was growing by fixing its tail:

This behavior has changed. The snake no longer needs to fix its tail to grow:

Also, the growth rate has been fine-tuned for better gameplay.

Speed Change

With this update, the game starts with a faster snake. Each food slows the snake down to a reasonable speed.

This helps the player to skip the early game faster and play the late game more carefully.

Collision

Increased the collision tolerance so that the player doesn't lose because of a pixel. I also loosened the turn delayer with the same tolerance for consistency. This leads to more "juicy" turns: