Added push and pop commands

- Also added an iris_rom_init command to cut down on duplicated code in tools
sim, img, and dbg now use iris_rom_init
Added push and pop support
Fixed the stack so that index zero is used correctly

- The index during a push is incremented first and that address is used.
- When performing a pop the address is decremented after extracting the current
  value.
Added push and pop to the assembler