A few fixes to the build system (mostly for ocamlbuild)

 * vars.mli was mentionning Term instead of Constr, leading to a dep cycle
 * Having a file named toplevel/toplevel.ml isn't a good idea when we also
   have a toplevel/toplevel.mllib that ought to produce a toplevel.cma.
   We rename toplevel.ml into Coqloop.ml
 * Extra cleanup of toplevel.mllib :
   - Ppextra isn't anywhere around (?!)
   - Ppvernac was mentionned twice, and rather belongs to printing.mllib anyway
   - Vernacexpr is a .mli and shouldn't appear in an .mllib
 * During the link, printing.cma now comes after parsing.cma (Ppvernac uses Egramml)
 * A few extra -threads in ocamlbuild files (btw, it's a bit sad to need -thread
   for coqchk).
Get rid of messages "emitting ..." when compiling .v files

 If these messages are still relevent to somebody, feel free to
 restore them in -debug or any non-default mode of your choice.