Code Improvements

[brholtkamp]

• Abstract out collision events for enemies into CollideHorizontally(point) and CollideVertically(point) to make the code DRY. Currently Goombas and Koopas end up doing the same logic for determine collision types
• Make the camera no longer allow the player to go backwards. This is expected behaviour based off the original game.
• Convert any existing foreach checks into Linq style checks for collision
• Look into the possibility of extracting collision logic into ActorController
• Move currentInvulnerabilityDuration into FixedUpdate instead of Update