Make ist (interp_sign) available to TACTIC EXTEND code

In order to do so I had to move the data base of tactics to
tacinterp (from tacintern).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16540 85f007b7-540e-0410-9357-904b9bb8a0f7