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/.ocamlformat

@@ -0,0 +1,2 @@
+profile = janestreet
+version = 0.26.2