Uniformizing generic argument types.

Now, instead of having three unrelated types describing a dynamic
type at each level (raw, glob, top), we have a "('a, 'b, 'c) genarg_type"
whose parameters describe the reified type at each level.

This has various advantages:

- No more code duplication to handle the three level separately;
- Safer code: one is not authorized to mix unrelated types when what
was morally expected was a genarg_type.
- Each level-specialized representation can be accessed through
well-typed projections: rawwit, glbwit and topwit.

Documenting a bit Genarg b.t.w.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16564 85f007b7-540e-0410-9357-904b9bb8a0f7