WHY_HERE

New lib for the slicer.
We need it to be potentially called as up as possible
E.G. we want to add a debug tracer of the slicing process in qmltop,
to have a interactive loop with it.

A test with a slicing to a double ocaml side will also take place here.

Since this version of the slicer works on new ast,
there is no other dependancies in OPA.

This lib contains only the kernel of the slicing process,
the opa-specific traitements are still in opa.

With opageneral, anyway there is no more problems to have the slicer lib 
here w.r.t the duration of the build process (thanks to Louis)

We hope as well that in this way, ast and typer changes will be corrected
in this lib by the commiters without asking them to be brave enough to go 
to opa repository.