finish project eight backend compiler

- modify existing implementation to use variant types for a more
  accurate representation of the vm byte code
- switch to fold from recursion for the main translate function
- use separate modules for translating different vm commands
- move static arithmetic command translations to a map

compiler/backend/test/dune

@@ -0,0 +1,3 @@
+(tests
+ (names test_backend)
+ (libraries backend ounit2))

compiler/backend/test/test_backend.ml

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+open OUnit2
+open Backend.Ast
+open Backend.Translate
+
+let peq case value = assert_equal value (parse case)
+
+let test_parse_a_command _ =
+  peq "add" (ArithmeticCommand Add);
+  peq "sub" (ArithmeticCommand Sub);
+  peq "neg" (ArithmeticCommand Neg);
+  peq "eq" (ArithmeticCommand Eq);
+  peq "gt" (ArithmeticCommand Gt);
+  peq "lt" (ArithmeticCommand Lt);
+  peq "and" (ArithmeticCommand And);
+  peq "or" (ArithmeticCommand Or);
+  peq "not" (ArithmeticCommand Not)
+;;
+
+let test_parse_m_command _ =
+  peq "pop local 12" (MemoryAccessCommand (Pop, Local, 12));
+  peq "push argument 4" (MemoryAccessCommand (Push, Argument, 4))
+;;
+
+let suite =
+  "suite"
+  >::: [ "A Instruction" >:: test_parse_a_command
+       ; "M Instruction" >:: test_parse_m_command
+       ]
+;;
+
+let () = run_test_tt_main suite