Rename 7_virtual_machine_1_stack_arithmetic/.

books/nand2tetris/projects/
(7_virtual_machine_1_stack_arithmetic -> 7_8_virtual_machine)/
	-Rename the `7_virtual_machine_1_stack_arithmetic` directory to
	`7_8_virtual_machine`, since chapters 7 and 8 deal with the same
	project (a virtual machine translator) and will thus be
	implemented in the same package.