The typechecker fails with an exception on functor applications

[shym]

On:

val tricky : Set.Make(Int).t -> bool

the typechecker fails with:

gospel: internal error, uncaught exception:
        Invalid_argument("Ppxlib.Longident.flatten")
        Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
        Called from Ppxlib__Longident.flatten_exn in file "src/longident.ml" (inlined), line 34, characters 22-33
        Called from Gospel__Typing.ty_of_core in file "src/typing.ml", line 101, characters 39-70
        Called from Gospel__Typing.val_parse_core_type in file "src/typing.ml", line 766, characters 8-25
        Called from Gospel__Typing.process_val in file "src/typing.ml", line 964, characters 18-49
        Called from Gospel__Typing.process_sig_item.process_sig_item in file "src/typing.ml", line 1329, characters 31-67
        Called from Gospel__Typing.process_sig_item in file "src/typing.ml", line 1361, characters 23-51
        Called from Gospel__Typing.type_sig_item in file "src/typing.ml", line 1366, characters 15-49

[shym]

Proposal of fix:

• add a new “Functor application not supported” error,
• wrap all calls to Longident.flatten_exn to report that error if need be.