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, 1 insertions, 0 deletions

diff --git a/checker/check.mllib b/checker/check.mllib
index 2fa4d5797..1925477e0 100644
--- a/checker/check.mllib
+++ b/checker/check.mllib
@@ -35,6 +35,7 @@ Segmenttree
 Unicodetable
 Unicode
 Errors
+CWarnings
 CEphemeron
 Future
 CUnix