Add all base classes for project 9, a simulation.

9_high_level_language/(Main, Particle, Point, Simulation, Square).jack
	-Add all base classes for my choice of project for chapter 9, a
	dead simple 2D particle physics simulation.
Complete chapter 9 project.

9_high_level_language/(Particle, Simulation).jack
	-Complete my chapter 9 project.
	-Complete `Simulation.run()`; add logic to handle particles that
	move past the borders of the screen.
	-Define `Particle` methods `getX()`, `getY()`, `setX()`,
	`setY()`. `setDx()`, and `setDy()`.
Document the entirety of project 9.

9_high_level_language/(Main, Particle, Point, Simulation, Square).jack
	-Add module-level documentation comments, summarizing the
	contents of each file.
Merge branch 'books/nand2tetris.9_high_level_language' into books/nand2tetris

books/nand2tetris/projects/9_high_level_language/
	-Implement an improvised project for chapter 9, "High Level
	Language", of nand2tetris.