crash on enum-like signatures due to unsafe use of succ()

[hackedy]

If I define an enum-like type

bit : Type

zero : bit
one : bit

then autosubst-ocaml crashes with the error message

Fatal error: exception Invalid_argument("[ocamlgraph] succ")
Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
Called from Graph__Blocks.Unlabeled.succ in file "src/blocks.ml", line 235, characters 28-71
Called from Autosubst_lib__AssocList.map.(fun) in file "lib/assocList.ml", line 39, characters 30-35
Called from Stdlib__List.map in file "list.ml", line 92, characters 20-23
Called from Autosubst_lib__SigAnalyzer.build_signature in file "lib/sigAnalyzer.ml", line 82, characters 18-53
Called from Stdlib__Result.map in file "result.ml", line 25, characters 32-37
Called from Monadic__Result.MakeT.ResultMonad.bind in file "monadic/library/result.ml", line 31, characters 24-33
Called from Autosubst_lib__Program.main in file "lib/program.ml", line 186, characters 19-51
Called from Stdlib__Result.map in file "result.ml", line 25, characters 32-37
Called from Monadic__Result.MakeT.ResultMonad.bind in file "monadic/library/result.ml", line 31, characters 24-33
Called from Stdlib__Result.map in file "result.ml", line 25, characters 32-37
Called from Monadic__Result.MakeT.ResultMonad.bind in file "monadic/library/result.ml", line 31, characters 24-33
Called from Dune__exe__Main.main in file "bin/main.ml", line 6, characters 12-35
Called from Dune__exe__Main in file "bin/main.ml", line 10, characters 9-16

If, however, the enum-like type appears as a parameter of a constructor in some other type, then there is no crash.

bit : Type

zero : bit
one : bit

expr : Type
const : bit -> expr
and : expr -> expr -> expr

I guess there is some use of the succ() function while analyzing the signature that does not guard against the possibility of a symbol having no successors of some kind.

@bamarsha found this bug.