Splitting Term into five unrelated interfaces:

1. sorts.ml: A small file utility for sorts;

2. constr.ml: Really low-level terms, essentially kind_of_constr, smart
constructor and basic operators;

3. vars.ml: Everything related to term variables, that is, occurences
and substitution;

4. context.ml: Rel/Named context and all that;

5. term.ml: derived utility operations on terms; also includes constr.ml
up to some renaming, and acts as a compatibility layer, to be deprecated.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16462 85f007b7-540e-0410-9357-904b9bb8a0f7
Merging Context and Sign.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16463 85f007b7-540e-0410-9357-904b9bb8a0f7