A new infrastructure for warnings.

On the user side, coqtop and coqc take a list of warning names or categories
after -w. No prefix means activate the warning, a "-" prefix means deactivate
it, and "+" means turn the warning into an error. Special categories include
"all", and "default" which contains the warnings enabled by default.

We also provide a vernacular Set Warnings which takes the same flags as argument.

Note that coqc now prints warnings.

The name and category of a warning are printed with the warning itself.

On the developer side, Feedback.msg_warning is still accessible, but the
recommended way to print a warning is in two steps:

1) create it by:

let warn_my_warning =
  CWarnings.create ~name:"my-warning" ~category:"my-category"
    (fun args -> Pp.strbrk ...)

2) print it by:

warn_my_warning args

1 files changed, 3 insertions, 4 deletions

diff --git a/kernel/vconv.ml b/kernel/vconv.ml
index 53db6f5bee..c729a6ce29 100644
--- a/kernel/vconv.ml
+++ b/kernel/vconv.ml
@@ -185,10 +185,9 @@ let vm_conv_gen cv_pb env univs t1 t2 =
     let v2 = val_of_constr env t2 in
     fst (conv_val env cv_pb (nb_rel env) v1 v2 univs)
   with Not_found | Invalid_argument _ ->
-    (Feedback.msg_warning
-      (Pp.str "Bytecode compilation failed, falling back to default conversion");
-     Reduction.generic_conv cv_pb ~l2r:false (fun _ -> None)
-			    full_transparent_state env univs t1 t2)
+    warn_bytecode_compiler_failed ();
+    Reduction.generic_conv cv_pb ~l2r:false (fun _ -> None)
+      full_transparent_state env univs t1 t2
 
 let vm_conv cv_pb env t1 t2 =
   let univs = Environ.universes env in