* src/tgba/state.hh: Define state_set and shared_state_set.
* src/tgba/taatgba.cc, src/tgba/taatgba.hh: Rename the existing
state_set (that inherits from spot::state) as set_state.
* src/tgba/tgbakvcomplement.cc: Use shared_state_set instead
of state_set.
* src/tgbaalgos/minimize.cc (state_set): Rename as...
(build_state_set): ... this.