This commit replaces the ue2::unordered_{set,map} types with their STL
versions, with some new hashing utilities in util/hash.h. The new types
ue2_unordered_set<T> and ue2_unordered_map<Key, T> default to using the
ue2_hasher.
The header util/ue2_containers.h has been removed, and the flat_set/map
containers moved to util/flat_containers.h.
As somMayGoBackwards() operates on a copy of the graph where virtual
starts have been collapsed on to startDs, we need to be careful not to
create parallel edges.
This changes all the execute_graph() interfaces so that instead of
mutating a std::set of vertices, they accept an initial flat_set of
states and return a resultant flat_set of states after execution.
(Note that internally execute_graph() still uses bitsets)
This is both faster and more flexible.
