Remove Event::None, add Event::Unknown, panic on unreachable Events
Remove Event::None, add Event::Unknown, panic on unreachable Events

* `Event::None` can be replaced by `Option::None`
* `Event::Unknown` is for events not recognized by rust-sdl2.
  New events are possible if a newer version of libSDL2 is linked.
* Certain events such as First and Last cannot be received.
  If they are received, it's a runtime error.

PistonDevelopers/rust-sdl2,caldwell/rust-sdl2,nukep/rust-sdl2,Rust-SDL2/rust-sdl2,Rust-SDL2/rust-sdl2,crumblingstatue/rust-sdl2,inrustwetrust/rust-sdl2,wilsoniya/rust-sdl2,AngryLawyer/rust-sdl2,tempbottle/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,bvssvni/rust-sdl2,Cobrand/rust-sdl2,oldmanmike/rust-sdl2,bfops/rust-sdl2,beni55/rust-sdl2,Rust-SDL2/rust-sdl2,vadixidav/rust-sdl2