Sgi::hash_map<const formula*, ...>.

* src/misc/hash.hh: New file.
* src/misc/Makefile.am (misc_HEADERS): Add it.
* src/ltlvisit/dotty.cc (dotty_visitor::map): Use a hash_map instead
of a map.
* src/tgba/bdddict.hh (bdd_dict::fv_map, bdd_dict::vf_map,
bdd_dict::ref_set, bdd_dict::var_map): Define as hash_map or
hash_set.
* src/tgbaalgos/ltl2tgba_fm.cc (translate_dict::fv_map,
translate_dict::vf_map): Likewise.
* src/tgba/tgbabddconcretefactory.hh
(tgba_bdd_concrete_factory::acc_map_): Likewise.
* src/tgba/tgbatba.hh, src/tgbaalgos/reachiter.hh: Include <map>.