Speed up SPEC_ALL

Note that reusing set from hyp_frees is actually slower.
times using `time bin/build`
before
real 10m37.510s
user 21m43.869s
sys 3m12.949s
reusing set from hyp_frees
real 10m40.847s
user    21m49.628s
sys     3m16.913s
without reusing set from hyp_frees
real    10m29.285s
user    21m9.849s
sys 3m10.606s

Author: ordinarymath

src/1/Drule.sml

@@ -782,7 +782,7 @@ in
       if is_forall (concl th) then
          let
             val (hvs, con) = (HOLset.listItems ## I) (hyp_frees th, concl th)
-            val fvs = free_vars con
+            val fvs = HOLset.listItems (FVL [con] empty_tmset)
             val vars = fst (strip_forall con)
          in
             SPECL (snd (itlist varyAcc vars (hvs @ fvs, []))) th