Right now, destroy() just executes "delete this".  But in a later
version, we will rewrite tgba_explicit so that it does not
allocate new states (and the destroy() method for explicit state
will do nothing).

* src/tgba/state.hh (state::destroy): New method, to replace
state::~state() in the future.
(shared_state_deleter): New function.
* src/evtgba/product.cc, src/evtgbaalgos/reachiter.cc,
src/evtgbaalgos/save.cc, src/evtgbaalgos/tgba2evtgba.cc,
src/tgba/tgba.cc, src/tgba/tgbaproduct.cc, src/tgba/tgbareduc.cc,
src/tgba/tgbasafracomplement.cc, src/tgba/tgbasgba.cc,
src/tgba/tgbatba.cc, src/tgba/tgbaunion.cc, src/tgba/wdbacomp.cc,
src/tgbaalgos/cutscc.cc, src/tgbaalgos/emptiness.cc,
src/tgbaalgos/gtec/ce.cc, src/tgbaalgos/gtec/explscc.cc,
src/tgbaalgos/gtec/gtec.cc, src/tgbaalgos/gtec/nsheap.cc,
src/tgbaalgos/gv04.cc, src/tgbaalgos/magic.cc,
src/tgbaalgos/minimize.cc, src/tgbaalgos/ndfs_result.hxx,
src/tgbaalgos/neverclaim.cc, src/tgbaalgos/powerset.hh,
src/tgbaalgos/reachiter.cc, src/tgbaalgos/reducerun.cc,
src/tgbaalgos/reductgba_sim.cc,
src/tgbaalgos/reductgba_sim_del.cc, src/tgbaalgos/replayrun.cc,
src/tgbaalgos/safety.cc, src/tgbaalgos/save.cc,
src/tgbaalgos/scc.cc, src/tgbaalgos/se05.cc,
src/tgbaalgos/tau03.cc, src/tgbaalgos/tau03opt.cc: Adjust to call
"s->destroy()" instead of "delete s".
* src/saba/sabacomplementtgba.cc, src/tgba/tgbakvcomplement.cc:
Pass shared_state_deleter to the shared_ptr constructor, so that
it calls destroy() instead of delete.