Add game entity image data include files

These contain scaled versions of all game entity images, to be used on the
AVR port.
game: event counter reports ints instead of floats

Floats not showing up properly on AVR.  The important numbers are much
larger than 1.
game entity: Replace dx and dy with a speed value

All entities move horizontally or vertically depending on the type,
so there is no need to keep both dx and dy.

Game data: 1669 -> 1451 bytes
Game entity: 13 -> 11 bytes
main: Infinite loop at end for embedded systems

This way we know if the program ended correctly.
Game entity: store array index of every entity

This will be used to look up sprite allocations later.

Game data size: 1451 -> 1560 bytes
Game entity size: 11 -> 12 bytes
system: Increase UART baud rate to 57600
screen: Remove blit list on AVR

I had thought that clearing the contents of struct blit would set the size
to zero, but apparently each struct still gets a byte allocated for it.

After removing the blit struct, the size of Screen on AVR dropped from 364
byte to 4 bytes.
images: add image loader for AVR

Loads images from flash (program) memory into ChronoCube VRAM.
main: print size and address of Screen and Image
game: print address of allocated game data