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/bin/dune

@@ -0,0 +1,5 @@
+(executable
+ (modes byte exe)
+ (name compiler)
+ (public_name compiler)
+ (libraries backend core core_unix.command_unix))