add debug option for compiler builds to get nice backtraces on exceptions

(This is not meant for end-users, but to improve the development experience.)

Before:

OCAMLRUNPARAM=b ./compiler/hepts.byte -mod repro -node foo -exec repro_c/sim
Fatal error: exception Not_found
Called from unknown location
Called from unknown location

After:

OCAMLRUNPARAM=b ./compiler/hepts.byte -mod repro -node foo -exec repro_c/sim
Fatal error: exception Not_found
Raised at Stdlib__Map.Make.find in file "map.ml", line 137, characters 10-25
Called from Hepts.main in file "main/hepts.ml", line 374, characters 18-90
Called from Hepts in file "main/hepts.ml", line 829, characters 4-11