nand2tetris chapter 4: add notes.

books/nand2tetris/notes/4_machine_language.md
	-Add notes for the 4th chapter of nand2tetris, "Machine
	Language".
nand2tetris project 4: add Mult.asm.

books/nand2tetris/projects/4_machine_language/Mult.asm
	-Add `Mult.asm`, an implementation of multiplication.
nand2tetris project 4: add Termial.asm.

books/nand2tetris/projects/4_machine_language/Termial.asm
	-Add an assembly program that calculate the termial function of
	a function.
nand2tetris project 4: add Division.asm.

books/nand2tetris/projects/4_machine_language/Division.asm
	-Add an assembly program that calculates the quotient of two
	positive numbers.
nand2tetris project 4: add Fill.asm.

books/nand2tetris/projects/4_machine_language/Fill.asm
	-Add a program to fill/clear the screen on user input.
Merge branch 'books/nand2tetris.4_machine_language' into books/nand2tetris

books/nand2tetris/
(notes/4_machine_language.md, projects/4_machine_language/)
	-Complete chapter 4, "machine language", of nand2tetris.