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/lib/dune

@@ -0,0 +1,8 @@
+(ocamllex lexer)
+
+(menhir
+ (modules parser))
+
+(library
+ (name backend)
+ (libraries core))