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/dune-project

@@ -0,0 +1,2 @@
+(lang dune 3.13)
+(using menhir 3.0)