Fix project 4 Fill.asm bugs.

books/nand2tetris/projects/4_machine_language/Fill.asm
	-Fix a bug in `Fill.asm`, completed for project 4
	(0342eb6), which prevented the screen from getting cleared
	(overlooked in refactoring).
Support swapped operands in C instructions.

books/nand2tetris/projects/6_assembler/assembler/instruction.py
	-Modify `CInstr.to_binary()`, automating the process of adding
	any operations on the `A` register by simply taking the `M`
	register equivalent, replacing `M` with `A`, and modifying the
	`a` bit.
	-Add support for swapped operands in addition operations (eg,
	`D + M` and not only `M + D`).

books/nand2tetris/projects/6_assembler/assembler/parser.py