Make state_explicit instances persistent objects.
* src/tgba/tgbaexplicit.cc, src/tgba/tgbaexplicit.hh: Merge state_explicit and tgba_explicit::state. In the past, state_explicit was a small object encapsulating a pointer to the persistent tgba_explicit::state; and we used to clone() and destroy() a lot of state_explicit instance. Now state_explicit is persistent, and clone() and destroy() have no effects. * src/tgba/tgbareduce.cc: Adjust, since this inherits from tgbaexplicit and uses the internals of state_explicit. * src/tgbatest/reductgba.cc: Fix deletion order for automata. * src/tgba/tgba.hh (last_support_conditions_input_, last_support_variables_input_): Make these protected, so they can be zeroed by tgba_explicit.