Made a couple of changes to gameplay to make the game fairer on later levels for the player and keep the flow of the game fast.

- Player now has a speed increment dependent on which wave number they're on (basically the player becomes faster with each level). Stripping out the speed code means I took out the speed penalty when the player is carrying a goblin. I was planning to put this back in but actually the game flows a lot better without it, so I'm planning on keeping that for now.

- Goblins now instantly leave the playing field if they are stunned outside the field area. I briefly
implemented this before but it seemed buggy. Now realised it was because if a goblin was flying overhead
they'd fall back down instantly, so I've coded an exception that they remain stunned if a goblin collides with
them that is in their flying state.