Note that `Some ()` goes to constant branch
For constant branch, we pretty much know it is. undefined or not
while `Some (x : unit)` goes to type based analysis
For type based, if it is predefined unit, done, otherwise need check its constructor description

turn Fld_na to Fld_na of string for debugging

- transl_extension_path will be a temporary thing, it is a string in BS
- in native, for non-const, it compares (Fld 0) with `transl_extension_path`, for const, it use itself to compare against `transl_extension_path`
- for bs, in both cases, it calls `extension_slot_eq exn transl_extension_path`

…e sets per backend

…mpiling classes

…ined in bs

…Cheng

… fails to implement its contract